I am so sorry, if my comments look a little bit "dumb".

No i realized, that the File with the translation has to go to the "/root/libphutil/src/extensions" Folder, some Values are translated in the ui, but others aren't. Is there a file, where i can find all strings, to translate it? Or is there another way to find out how to translate a specific string?

Let me add some note: A changing of the "Translation" - Dropdown in the individual account settings never makes a change. So it is not important if i choose german or something else. That is not influencing my request, but is that a normal behaviour? Or did i maybe make a mistake when i set up the system?

In P1821#4300, @devurandom wrote:

The default language can be changed using following patch (cf. T8916):

diff --git a/src/applications/settings/panel/PhabricatorAccountSettingsPanel.php b/src/applications/settings/panel/PhabricatorAccountSettingsPanel.php
index 72a2252..b233ba0 100644
--- a/src/applications/settings/panel/PhabricatorAccountSettingsPanel.php
+++ b/src/applications/settings/panel/PhabricatorAccountSettingsPanel.php
@@ -72,7 +72,7 @@ final class PhabricatorAccountSettingsPanel extends PhabricatorSettingsPanel {
 
     asort($translations);
     // TODO: Implement "locale.default" and use it here.
-    $default = 'en_US';
+    $default = 'en_P*';
     $translations = array(
       '' => pht('Server Default: %s', $locales[$default]->getLocaleName()),
     ) + $translations;
diff --git a/src/infrastructure/env/PhabricatorEnv.php b/src/infrastructure/env/PhabricatorEnv.php
index d5c4c34..3c09d0b 100644
--- a/src/infrastructure/env/PhabricatorEnv.php
+++ b/src/infrastructure/env/PhabricatorEnv.php
@@ -126,7 +126,7 @@ final class PhabricatorEnv extends Phobject {
 
     // TODO: Add a "locale.default" config option once we have some reasonable
     // defaults which aren't silly nonsense.
-    self::setLocaleCode('en_US');
+    self::setLocaleCode('en_P*');
   }
 
   public static function beginScopedLocale($locale_code) {

This is the full code of the file in my case.

Do you mean the patch, added by devurandom ? The first part of the patch ($default...) is not existing in my PhabricatorAccountSettingsPanel.php