Allow PhutilTranslator::translate() to return defaults

Summary: Allow PhutilTranslator::translate() to return defaults

Test Plan: Just check some strings returned correctly.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, Zolli, epriestley

Projects: Localization

Maniphest Tasks: T6845

Differential Revision: https://secure.phabricator.com/D11121