diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -83,6 +83,7 @@
     'PhutilAWSFuture' => 'future/aws/PhutilAWSFuture.php',
     'PhutilAWSS3Future' => 'future/aws/PhutilAWSS3Future.php',
     'PhutilAggregateException' => 'error/PhutilAggregateException.php',
+    'PhutilAllCapsEnglishLocale' => 'internationalization/locales/PhutilAllCapsEnglishLocale.php',
     'PhutilAmazonAuthAdapter' => 'auth/PhutilAmazonAuthAdapter.php',
     'PhutilArgumentParser' => 'parser/argument/PhutilArgumentParser.php',
     'PhutilArgumentParserException' => 'parser/argument/exception/PhutilArgumentParserException.php',
@@ -106,6 +107,7 @@
     'PhutilBitbucketAuthAdapter' => 'auth/PhutilBitbucketAuthAdapter.php',
     'PhutilBootloader' => 'moduleutils/PhutilBootloader.php',
     'PhutilBootloaderException' => 'moduleutils/PhutilBootloaderException.php',
+    'PhutilBritishEnglishLocale' => 'internationalization/locales/PhutilBritishEnglishLocale.php',
     'PhutilBufferedIterator' => 'utils/PhutilBufferedIterator.php',
     'PhutilBufferedIteratorTestCase' => 'utils/__tests__/PhutilBufferedIteratorTestCase.php',
     'PhutilBugtraqParser' => 'parser/PhutilBugtraqParser.php',
@@ -133,6 +135,7 @@
     'PhutilConsoleWrapTestCase' => 'console/__tests__/PhutilConsoleWrapTestCase.php',
     'PhutilContextFreeGrammar' => 'grammar/PhutilContextFreeGrammar.php',
     'PhutilCsprintfTestCase' => 'xsprintf/__tests__/PhutilCsprintfTestCase.php',
+    'PhutilCzechLocale' => 'internationalization/locales/PhutilCzechLocale.php',
     'PhutilDaemon' => 'daemon/PhutilDaemon.php',
     'PhutilDaemonOverseer' => 'daemon/PhutilDaemonOverseer.php',
     'PhutilDefaultSyntaxHighlighter' => 'markup/syntax/highlighter/PhutilDefaultSyntaxHighlighter.php',
@@ -211,6 +214,7 @@
     'PhutilLibraryMapBuilder' => 'moduleutils/PhutilLibraryMapBuilder.php',
     'PhutilLibraryTestCase' => '__tests__/PhutilLibraryTestCase.php',
     'PhutilLipsumContextFreeGrammar' => 'grammar/PhutilLipsumContextFreeGrammar.php',
+    'PhutilLocale' => 'internationalization/PhutilLocale.php',
     'PhutilLock' => 'filesystem/PhutilLock.php',
     'PhutilLockException' => 'filesystem/PhutilLockException.php',
     'PhutilLogFileChannel' => 'channel/PhutilLogFileChannel.php',
@@ -258,6 +262,7 @@
     'PhutilQueryStringParser' => 'parser/PhutilQueryStringParser.php',
     'PhutilQueryStringParserTestCase' => 'parser/__tests__/PhutilQueryStringParserTestCase.php',
     'PhutilRainbowSyntaxHighlighter' => 'markup/syntax/highlighter/PhutilRainbowSyntaxHighlighter.php',
+    'PhutilRawEnglishLocale' => 'internationalization/locales/PhutilRawEnglishLocale.php',
     'PhutilReadableSerializer' => 'readableserializer/PhutilReadableSerializer.php',
     'PhutilReadableSerializerTestCase' => 'readableserializer/__tests__/PhutilReadableSerializerTestCase.php',
     'PhutilRealNameContextFreeGrammar' => 'grammar/PhutilRealNameContextFreeGrammar.php',
@@ -313,6 +318,7 @@
     'PhutilTestCase' => 'infrastructure/testing/PhutilTestCase.php',
     'PhutilTestPhobject' => 'object/__tests__/PhutilTestPhobject.php',
     'PhutilTortureTestDaemon' => 'daemon/torture/PhutilTortureTestDaemon.php',
+    'PhutilTranslation' => 'internationalization/PhutilTranslation.php',
     'PhutilTranslator' => 'internationalization/PhutilTranslator.php',
     'PhutilTranslatorTestCase' => 'internationalization/__tests__/PhutilTranslatorTestCase.php',
     'PhutilTwitchAuthAdapter' => 'auth/PhutilTwitchAuthAdapter.php',
@@ -326,6 +332,7 @@
     'PhutilTypeSpecTestCase' => 'parser/__tests__/PhutilTypeSpecTestCase.php',
     'PhutilURI' => 'parser/PhutilURI.php',
     'PhutilURITestCase' => 'parser/__tests__/PhutilURITestCase.php',
+    'PhutilUSEnglishLocale' => 'internationalization/locales/PhutilUSEnglishLocale.php',
     'PhutilUTF8StringTruncator' => 'utils/PhutilUTF8StringTruncator.php',
     'PhutilUTF8TestCase' => 'utils/__tests__/PhutilUTF8TestCase.php',
     'PhutilUnknownSymbolParserGeneratorException' => 'parser/generator/exception/PhutilUnknownSymbolParserGeneratorException.php',
@@ -333,6 +340,7 @@
     'PhutilUnreachableTerminalParserGeneratorException' => 'parser/generator/exception/PhutilUnreachableTerminalParserGeneratorException.php',
     'PhutilUrisprintfTestCase' => 'xsprintf/__tests__/PhutilUrisprintfTestCase.php',
     'PhutilUtilsTestCase' => 'utils/__tests__/PhutilUtilsTestCase.php',
+    'PhutilVeryWowEnglishLocale' => 'internationalization/locales/PhutilVeryWowEnglishLocale.php',
     'PhutilWordPressAuthAdapter' => 'auth/PhutilWordPressAuthAdapter.php',
     'PhutilWordPressFuture' => 'future/wordpress/PhutilWordPressFuture.php',
     'PhutilXHPASTBinary' => 'parser/xhpast/bin/PhutilXHPASTBinary.php',
@@ -532,6 +540,7 @@
     'PhutilAWSFuture' => 'FutureProxy',
     'PhutilAWSS3Future' => 'PhutilAWSFuture',
     'PhutilAggregateException' => 'Exception',
+    'PhutilAllCapsEnglishLocale' => 'PhutilLocale',
     'PhutilAmazonAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilArgumentParserException' => 'Exception',
     'PhutilArgumentParserTestCase' => 'PhutilTestCase',
@@ -554,6 +563,7 @@
     'PhutilAuthUserAbortedException' => 'PhutilAuthException',
     'PhutilBitbucketAuthAdapter' => 'PhutilOAuth1AuthAdapter',
     'PhutilBootloaderException' => 'Exception',
+    'PhutilBritishEnglishLocale' => 'PhutilLocale',
     'PhutilBufferedIterator' => 'Iterator',
     'PhutilBufferedIteratorTestCase' => 'PhutilTestCase',
     'PhutilBugtraqParserTestCase' => 'PhutilTestCase',
@@ -573,6 +583,7 @@
     'PhutilConsoleTable' => 'Phobject',
     'PhutilConsoleWrapTestCase' => 'PhutilTestCase',
     'PhutilCsprintfTestCase' => 'PhutilTestCase',
+    'PhutilCzechLocale' => 'PhutilLocale',
     'PhutilDefaultSyntaxHighlighterEngine' => 'PhutilSyntaxHighlighterEngine',
     'PhutilDefaultSyntaxHighlighterEnginePygmentsFuture' => 'FutureProxy',
     'PhutilDefaultSyntaxHighlighterEngineTestCase' => 'PhutilTestCase',
@@ -626,6 +637,7 @@
     'PhutilLibraryConflictException' => 'Exception',
     'PhutilLibraryTestCase' => 'PhutilTestCase',
     'PhutilLipsumContextFreeGrammar' => 'PhutilContextFreeGrammar',
+    'PhutilLocale' => 'Phobject',
     'PhutilLockException' => 'Exception',
     'PhutilLogFileChannel' => 'PhutilChannelChannel',
     'PhutilLunarPhaseTestCase' => 'PhutilTestCase',
@@ -661,6 +673,7 @@
     'PhutilProxyException' => 'Exception',
     'PhutilPythonFragmentLexer' => 'PhutilLexer',
     'PhutilQueryStringParserTestCase' => 'PhutilTestCase',
+    'PhutilRawEnglishLocale' => 'PhutilLocale',
     'PhutilReadableSerializerTestCase' => 'PhutilTestCase',
     'PhutilRealNameContextFreeGrammar' => 'PhutilContextFreeGrammar',
     'PhutilRemarkupBoldRule' => 'PhutilRemarkupRule',
@@ -702,6 +715,7 @@
     'PhutilTestCase' => 'ArcanistPhutilTestCase',
     'PhutilTestPhobject' => 'Phobject',
     'PhutilTortureTestDaemon' => 'PhutilDaemon',
+    'PhutilTranslation' => 'Phobject',
     'PhutilTranslatorTestCase' => 'PhutilTestCase',
     'PhutilTwitchAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilTwitchFuture' => 'FutureProxy',
@@ -712,6 +726,7 @@
     'PhutilTypeMissingParametersException' => 'Exception',
     'PhutilTypeSpecTestCase' => 'PhutilTestCase',
     'PhutilURITestCase' => 'PhutilTestCase',
+    'PhutilUSEnglishLocale' => 'PhutilLocale',
     'PhutilUTF8StringTruncator' => 'Phobject',
     'PhutilUTF8TestCase' => 'PhutilTestCase',
     'PhutilUnknownSymbolParserGeneratorException' => 'PhutilParserGeneratorException',
@@ -719,6 +734,7 @@
     'PhutilUnreachableTerminalParserGeneratorException' => 'PhutilParserGeneratorException',
     'PhutilUrisprintfTestCase' => 'PhutilTestCase',
     'PhutilUtilsTestCase' => 'PhutilTestCase',
+    'PhutilVeryWowEnglishLocale' => 'PhutilLocale',
     'PhutilWordPressAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilWordPressFuture' => 'FutureProxy',
     'PhutilXHPASTSyntaxHighlighterFuture' => 'FutureProxy',
diff --git a/src/internationalization/PhutilLocale.php b/src/internationalization/PhutilLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/PhutilLocale.php
@@ -0,0 +1,211 @@
+<?php
+
+/**
+ * Defines a locale for translations.
+ *
+ * Examples might include "English (US)" or "Japanese".
+ */
+abstract class PhutilLocale extends Phobject {
+
+  /**
+   * Get the local identifier code, like "en_US".
+   *
+   * @return string Locale identifier code.
+   */
+  abstract public function getLocaleCode();
+
+
+  /**
+   * Get the human-readable locale name, like "English (US)".
+   *
+   * @return string Human-readable locale name.
+   */
+  abstract public function getLocaleName();
+
+
+  /**
+   * Set a fallback locale which can be used as a default if this locale is
+   * missing translations.
+   *
+   * For locales like "English (Great Britain)", missing translations can be
+   * sourced from "English (US)".
+   *
+   * @return string|null Locale code of fallback locale, or null if there is
+   *                     no fallback locale.
+   */
+  public function getFallbackLocaleCode() {
+    return null;
+  }
+
+
+  /**
+   * Flags a locale as silly, like "English (Pirate)".
+   *
+   * These locales are fun but disastrously inappropriate for serious
+   * businesses.
+   *
+   * @return bool True if this locale is silly.
+   */
+  public function isSillyLocale() {
+    return false;
+  }
+
+
+  /**
+   * Flags a locale as a testing locale, like "English (US, ALL CAPS)". These
+   * locales are useful for translation development, but not for normal users.
+   *
+   * @return bool True if this is a locale for testing or development.
+   */
+  public function isTestLocale() {
+    return false;
+  }
+
+
+  /**
+   * Indicates that the translator should post-process translations in this
+   * locale by calling @{method:didTranslateString}.
+   *
+   * Doing this incurs a performance penalty, and is not useful for most
+   * languages. However, it can be used to implement test translations like
+   * "English (US, ALL CAPS)".
+   *
+   * @return bool True to postprocess strings.
+   */
+  public function shouldPostProcessTranslations() {
+    return false;
+  }
+
+
+  /**
+   * Callback for post-processing translations.
+   *
+   * By default, this callback is not invoked. To activate it, return `true`
+   * from @{method:shouldPostProcessTranslations}. Activating this callback
+   * incurs a performance penalty.
+   *
+   * @param string The raw input pattern.
+   * @param string The selected translation pattern.
+   * @param list<wild> The raw input arguments.
+   * @param string The translated string.
+   * @return string Post-processed translation string.
+   */
+  public function didTranslateString(
+    $raw_pattern,
+    $translated_pattern,
+    array $args,
+    $result_text) {
+    return $result_text;
+  }
+
+
+  /**
+   * Load all available locales.
+   *
+   * @return map<string, PhutilLocale> Map from codes to locale objects.
+   */
+  public static function loadAllLocales() {
+    static $locales;
+    if ($locales === null) {
+      $objects = id(new PhutilSymbolLoader())
+        ->setAncestorClass(__CLASS__)
+        ->loadObjects();
+
+      $locale_map = array();
+      foreach ($objects as $object) {
+        $locale_code = $object->getLocaleCode();
+        if (empty($locale_map[$locale_code])) {
+          $locale_map[$locale_code] = $object;
+        } else {
+          throw new Exception(
+            pht(
+              'Two subclasses of "PhutilLocale" ("%s" and "%s") define '.
+              'locales with the same locale code ("%s"). Each locale must '.
+              'have a unique locale code.',
+              get_class($object),
+              get_class($locale_map[$locale_code]),
+              $locale_code));
+        }
+      }
+
+      foreach ($locale_map as $locale_code => $locale) {
+        $fallback_code = $locale->getFallbackLocaleCode();
+        if ($fallback_code !== null) {
+          if (empty($locale_map[$fallback_code])) {
+            throw new Exception(
+              pht(
+                'The locale "%s" has an invalid fallback locale code ("%s"). '.
+                'No locale class exists which defines this locale.',
+                get_class($locale),
+                $fallback_code));
+          }
+        }
+      }
+
+      foreach ($locale_map as $locale_code => $locale) {
+        $seen = array($locale_code => get_class($locale));
+        self::checkLocaleFallback($locale_map, $locale, $seen);
+      }
+
+      $locales = $locale_map;
+    }
+    return $locales;
+  }
+
+
+  /**
+   * Load a specific locale using a locale code.
+   *
+   * @param string Locale code.
+   * @return PhutilLocale Locale object.
+   */
+  public static function loadLocale($locale_code) {
+    $all_locales = self::loadAllLocales();
+    $locale = idx($all_locales, $locale_code);
+
+    if (!$locale) {
+      throw new Exception(
+        pht(
+          'There is no locale with the locale code "%s".',
+          $locale_code));
+    }
+
+    return $locale;
+  }
+
+
+  /**
+   * Recursively check locale fallbacks for cycles.
+   *
+   * @param map<string, PhutilLocale> Map of locales.
+   * @param PhutilLocale Current locale.
+   * @param map<string, string> Map of visited locales.
+   * @return void
+   */
+  private static function checkLocaleFallback(
+    array $map,
+    PhutilLocale $locale,
+    array $seen) {
+
+    $fallback_code = $locale->getFallbackLocaleCode();
+    if ($fallback_code === null) {
+      return;
+    }
+
+    if (isset($seen[$fallback_code])) {
+      $seen[] = get_class($locale);
+      $seen[] = pht('...');
+      throw new Exception(
+        pht(
+          'Locale "%s" is part of a cycle of locales which fall back on '.
+          'one another in a loop (%s). Locales which fall back on other '.
+          'locales must not loop.',
+          get_class($locale),
+          implode(' -> ', $seen)));
+    }
+
+    $seen[$fallback_code] = get_class($locale);
+    self::checkLocaleFallback($map, $map[$fallback_code], $seen);
+  }
+
+}
diff --git a/src/internationalization/PhutilTranslation.php b/src/internationalization/PhutilTranslation.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/PhutilTranslation.php
@@ -0,0 +1,89 @@
+<?php
+
+abstract class PhutilTranslation extends Phobject {
+
+  /**
+   * Get the locale code which this class translates text for, like
+   * "en_GB".
+   *
+   * This should correspond to a valid subclass of @{class:PhutilLocale}.
+   *
+   * @return string Locale code for this translation.
+   */
+  abstract public function getLocaleCode();
+
+
+  /**
+   * Return a map of all translations.
+   *
+   * @return map<string, wild> Map of raw strings to translations.
+   */
+  abstract protected function getTranslations();
+
+
+  /**
+   * Return a filtered map of all strings in this translation.
+   *
+   * Filters out empty/placeholder translations.
+   *
+   * @return map<string, wild> Map of raw strings to translations.
+   */
+  final public function getFilteredTranslations() {
+    $translations = $this->getTranslations();
+
+    foreach ($translations as $key => $translation) {
+      if ($translation === null) {
+        unset($translations[$key]);
+      }
+    }
+
+    return $translations;
+  }
+
+
+  /**
+   * Load all available translation objects.
+   *
+   * @return list<PhutilTranslation> List of available translation sources.
+   */
+  public static function loadAllTranslations() {
+    static $translations;
+    if ($translations === null) {
+      $translations = id(new PhutilSymbolLoader())
+        ->setAncestorClass(__CLASS__)
+        ->loadObjects();
+    }
+    return $translations;
+  }
+
+
+  /**
+   * Load the complete translation map for a locale.
+   *
+   * This will compile primary and fallback translations into a single
+   * translation map.
+   *
+   * @param string Locale code, like "en_US".
+   * @return map<string, wild> Map of all avialable translations.
+   */
+  public static function getTranslationMapForLocale($locale_code) {
+    $locale = PhutilLocale::loadLocale($locale_code);
+
+    $translations = self::loadAllTranslations();
+
+    $results = array();
+    foreach ($translations as $translation) {
+      if ($translation->getLocaleCode() == $locale_code) {
+        $results += $translation->getFilteredTranslations();
+      }
+    }
+
+    $fallback_code = $locale->getFallbackLocaleCode();
+    if ($fallback_code !== null) {
+      $results += self::getTranslationMapForLocale($fallback_code);
+    }
+
+    return $results;
+  }
+
+}
diff --git a/src/internationalization/PhutilTranslator.php b/src/internationalization/PhutilTranslator.php
--- a/src/internationalization/PhutilTranslator.php
+++ b/src/internationalization/PhutilTranslator.php
@@ -4,7 +4,9 @@
 
   static private $instance;
 
-  private $language = 'en';
+  private $locale;
+  private $localeCode;
+  private $shouldPostProcess;
   private $translations = array();
 
   public static function getInstance() {
@@ -18,8 +20,10 @@
     self::$instance = $instance;
   }
 
-  public function setLanguage($language) {
-    $this->language = $language;
+  public function setLocale(PhutilLocale $locale) {
+    $this->locale = $locale;
+    $this->localeCode = $locale->getLocaleCode();
+    $this->shouldPostProcess = $locale->shouldPostProcessTranslations();
     return $this;
   }
 
@@ -53,8 +57,8 @@
    * @param array Identifier in key, translation in value.
    * @return PhutilTranslator Provides fluent interface.
    */
-  public function addTranslations(array $translations) {
-    $this->translations = array_merge($this->translations, $translations);
+  public function setTranslations(array $translations) {
+    $this->translations = $translations;
     return $this;
   }
 
@@ -97,8 +101,12 @@
       $result = '[Invalid Translation!] '.$translation;
     }
 
-    if ($this->language == 'en-ac') {
-      $result = strtoupper($result);
+    if ($this->shouldPostProcess) {
+      $result = $this->locale->didTranslateString(
+        $text,
+        $translation,
+        $args,
+        $result);
     }
 
     if ($is_html) {
@@ -118,17 +126,23 @@
       $variant = $variant->getNumber();
     }
 
-    switch ($this->language) {
+    // TODO: Move these into PhutilLocale if benchmarks show we aren't
+    // eating too much of a performance cost.
 
-      case 'en':
-      case 'en-ac':
+    switch ($this->localeCode) {
+
+      case 'en_US':
+      case 'en_GB':
+      case 'en_W*':
+      case 'en_R*':
+      case 'en_A*':
         list($singular, $plural) = $translations;
         if ($variant == 1) {
           return $singular;
         }
         return $plural;
 
-      case 'cs':
+      case 'cs_CZ':
         if ($variant instanceof PhutilPerson) {
           list($male, $female) = $translations;
           if ($variant->getSex() == PhutilPerson::SEX_FEMALE) {
diff --git a/src/internationalization/__tests__/PhutilPhtTestCase.php b/src/internationalization/__tests__/PhutilPhtTestCase.php
--- a/src/internationalization/__tests__/PhutilPhtTestCase.php
+++ b/src/internationalization/__tests__/PhutilPhtTestCase.php
@@ -11,15 +11,18 @@
     $this->assertEqual('beer', pht('beer'));
     $this->assertEqual('1 beer(s)', pht('%d beer(s)', 1));
 
-    PhutilTranslator::getInstance()->addTranslations(
+    $english_locale = PhutilLocale::loadLocale('en_US');
+    PhutilTranslator::getInstance()->setLocale($english_locale);
+    PhutilTranslator::getInstance()->setTranslations(
       array(
         '%d beer(s)' => array('%d beer', '%d beers'),
       ));
 
     $this->assertEqual('1 beer', pht('%d beer(s)', 1));
 
-    PhutilTranslator::getInstance()->setLanguage('cs');
-    PhutilTranslator::getInstance()->addTranslations(
+    $czech_locale = PhutilLocale::loadLocale('cs_CZ');
+    PhutilTranslator::getInstance()->setLocale($czech_locale);
+    PhutilTranslator::getInstance()->setTranslations(
       array(
         '%d beer(s)' => array('%d pivo', '%d piva', '%d piv'),
       ));
diff --git a/src/internationalization/__tests__/PhutilTranslatorTestCase.php b/src/internationalization/__tests__/PhutilTranslatorTestCase.php
--- a/src/internationalization/__tests__/PhutilTranslatorTestCase.php
+++ b/src/internationalization/__tests__/PhutilTranslatorTestCase.php
@@ -3,8 +3,8 @@
 final class PhutilTranslatorTestCase extends PhutilTestCase {
 
   public function testEnglish() {
-    $translator = new PhutilTranslator();
-    $translator->addTranslations(
+    $translator = $this->newTranslator('en_US');
+    $translator->setTranslations(
       array(
         '%d line(s)' => array('%d line', '%d lines'),
         '%d char(s) on %d row(s)' => array(
@@ -31,11 +31,10 @@
   }
 
   public function testSingleVariant() {
-    $translator = new PhutilTranslator();
-    $translator->setLanguage('en');
+    $translator = $this->newTranslator('en_US');
 
     // In this translation, we have no alternatives for the first conversion.
-    $translator->addTranslations(
+    $translator->setTranslations(
       array(
         'Run the command %s %d time(s).' => array(
           array(
@@ -54,9 +53,8 @@
   }
 
   public function testCzech() {
-    $translator = new PhutilTranslator();
-    $translator->setLanguage('cs');
-    $translator->addTranslations(
+    $translator = $this->newTranslator('cs_CZ');
+    $translator->setTranslations(
       array(
         '%d beer(s)' => array('%d pivo', '%d piva', '%d piv'),
       ));
@@ -70,9 +68,8 @@
   }
 
   public function testPerson() {
-    $translator = new PhutilTranslator();
-    $translator->setLanguage('cs');
-    $translator->addTranslations(
+    $translator = $this->newTranslator('cs_CZ');
+    $translator->setTranslations(
       array(
         '%s wrote.' => array('%s napsal.', '%s napsala.'),
       ));
@@ -95,13 +92,13 @@
 
   public function testTranslateDate() {
     $date = new DateTime('2012-06-21');
+    $translator = $this->newTranslator('en_US');
 
-    $translator = new PhutilTranslator();
     $this->assertEqual('June', $translator->translateDate('F', $date));
     $this->assertEqual('June 21', $translator->translateDate('F d', $date));
     $this->assertEqual('F', $translator->translateDate('\F', $date));
 
-    $translator->addTranslations(
+    $translator->setTranslations(
       array(
         'June' => 'correct',
         '21' => 'wrong',
@@ -113,12 +110,17 @@
   }
 
   public function testSetInstance() {
-    PhutilTranslator::setInstance(new PhutilTranslator());
+    $english_translator = $this->newTranslator('en_US');
+
+    PhutilTranslator::setInstance($english_translator);
     $original = PhutilTranslator::getInstance();
     $this->assertEqual('color', pht('color'));
 
+    $british_locale = PhutilLocale::loadLocale('en_GB');
+
     $british = new PhutilTranslator();
-    $british->addTranslations(
+    $british->setLocale($british_locale);
+    $british->setTranslations(
       array(
         'color' => 'colour',
       ));
@@ -130,12 +132,13 @@
   }
 
   public function testFormatNumber() {
-    $translator = new PhutilTranslator();
+    $translator = $this->newTranslator('en_US');
+
     $this->assertEqual('1,234', $translator->formatNumber(1234));
     $this->assertEqual('1,234.5', $translator->formatNumber(1234.5, 1));
     $this->assertEqual('1,234.5678', $translator->formatNumber(1234.5678, 4));
 
-    $translator->addTranslations(
+    $translator->setTranslations(
       array(
         ',' => ' ',
         '.' => ',',
@@ -146,8 +149,9 @@
   }
 
   public function testNumberTranslations() {
-    $translator = new PhutilTranslator();
-    $translator->addTranslations(
+    $translator = $this->newTranslator('en_US');
+
+    $translator->setTranslations(
       array(
         '%s line(s)' => array('%s line', '%s lines'),
       ));
@@ -192,7 +196,8 @@
       ),
     );
 
-    $translator = new PhutilTranslator();
+    $translator = $this->newTranslator('en_US');
+
     foreach ($tests as $original => $translations) {
       foreach ($translations as $translation => $expect) {
         $valid = ($expect ? 'valid' : 'invalid');
@@ -208,7 +213,7 @@
     $string = '%s awoke <strong>suddenly</strong> at %s.';
     $when = '<4 AM>';
 
-    $translator = new PhutilTranslator();
+    $translator = $this->newTranslator('en_US');
 
     // When no components are HTML, everything is treated as a string.
     $who = '<span>Abraham</span>';
@@ -243,4 +248,10 @@
       $translation->getHTMLContent());
   }
 
+  private function newTranslator($locale_code) {
+    $locale = PhutilLocale::loadLocale($locale_code);
+    return id(new PhutilTranslator())
+      ->setLocale($locale);
+  }
+
 }
diff --git a/src/internationalization/locales/PhutilAllCapsEnglishLocale.php b/src/internationalization/locales/PhutilAllCapsEnglishLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/locales/PhutilAllCapsEnglishLocale.php
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * A test locale which transforms strings into uppercase.
+ *
+ * This can be helpful for identifying untranslated strings.
+ */
+final class PhutilAllCapsEnglishLocale extends PhutilLocale {
+
+  public function getLocaleCode() {
+    return 'en_A*';
+  }
+
+  public function getLocaleName() {
+    return pht('English (US, ALL CAPS)');
+  }
+
+  public function getFallbackLocaleCode() {
+    return 'en_US';
+  }
+
+  public function isTestLocale() {
+    return true;
+  }
+
+  public function shouldPostProcessTranslations() {
+    return true;
+  }
+
+  public function didTranslateString(
+    $raw_pattern,
+    $translated_pattern,
+    array $args,
+    $result_text) {
+    return phutil_utf8_strtoupper($result_text);
+  }
+
+}
diff --git a/src/internationalization/locales/PhutilBritishEnglishLocale.php b/src/internationalization/locales/PhutilBritishEnglishLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/locales/PhutilBritishEnglishLocale.php
@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * Serious, legitimate British English locale.
+ */
+final class PhutilBritishEnglishLocale extends PhutilLocale {
+
+  public function getLocaleCode() {
+    return 'en_GB';
+  }
+
+  public function getLocaleName() {
+    return pht('English (Great Britain)');
+  }
+
+  public function getFallbackLocaleCode() {
+    return 'en_US';
+  }
+
+}
diff --git a/src/internationalization/locales/PhutilCzechLocale.php b/src/internationalization/locales/PhutilCzechLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/locales/PhutilCzechLocale.php
@@ -0,0 +1,22 @@
+<?php
+
+/**
+ * Locale for "Czech (Czech Republic)".
+ */
+final class PhutilCzechLocale extends PhutilLocale {
+
+  public function getLocaleCode() {
+    return 'cs_CZ';
+  }
+
+  public function getLocaleName() {
+    return pht('Czech (Czech Republic)');
+  }
+
+  public function isTestLocale() {
+    // This doesn't have any translations yet so mark it as a test locale
+    // for now.
+    return true;
+  }
+
+}
diff --git a/src/internationalization/locales/PhutilRawEnglishLocale.php b/src/internationalization/locales/PhutilRawEnglishLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/locales/PhutilRawEnglishLocale.php
@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * A test locale which displays the raw strings which are fed into
+ * the translation engine.
+ */
+final class PhutilRawEnglishLocale extends PhutilLocale {
+
+  public function getLocaleCode() {
+    return 'en_R*';
+  }
+
+  public function getLocaleName() {
+    return pht('English (Raw Strings)');
+  }
+
+  public function isTestLocale() {
+    return true;
+  }
+
+}
diff --git a/src/internationalization/locales/PhutilUSEnglishLocale.php b/src/internationalization/locales/PhutilUSEnglishLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/locales/PhutilUSEnglishLocale.php
@@ -0,0 +1,16 @@
+<?php
+
+/**
+ * The default English locale.
+ */
+final class PhutilUSEnglishLocale extends PhutilLocale {
+
+  public function getLocaleCode() {
+    return 'en_US';
+  }
+
+  public function getLocaleName() {
+    return pht('English (US)');
+  }
+
+}
diff --git a/src/internationalization/locales/PhutilVeryWowEnglishLocale.php b/src/internationalization/locales/PhutilVeryWowEnglishLocale.php
new file mode 100644
--- /dev/null
+++ b/src/internationalization/locales/PhutilVeryWowEnglishLocale.php
@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * Much locale! Very English! Wow!
+ */
+final class PhutilVeryWowEnglishLocale extends PhutilLocale {
+
+  public function getLocaleCode() {
+    return 'en_W*';
+  }
+
+  public function getLocaleName() {
+    return pht('English (Very Wow)');
+  }
+
+  public function getFallbackLocaleCode() {
+    return 'en_US';
+  }
+
+  public function isSillyLocale() {
+    return true;
+  }
+
+}
diff --git a/src/internationalization/pht.php b/src/internationalization/pht.php
--- a/src/internationalization/pht.php
+++ b/src/internationalization/pht.php
@@ -3,8 +3,8 @@
 /**
  * Translate a string. It uses a translator set by
  * `PhutilTranslator::setInstance()` or translations specified by
- * `PhutilTranslator::getInstance()->addTranslations()` and language rules set
- * by `PhutilTranslator::getInstance()->setLanguage()`.
+ * `PhutilTranslator::getInstance()->setTranslations()` and language rules set
+ * by `PhutilTranslator::getInstance()->setLocale()`.
  *
  * @param string Translation identifier with `sprintf()` placeholders.
  * @param mixed Value to select the variant from (e.g. singular or plural).