Send mail to targets in the user's translation

Summary: Ref T6367.

Test Plan:

• Added and executed unit tests.
• Sent mail to A (en_US) and B (en_A*).
• Got one mail in English and one mail in ENGLISH.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6367

Differential Revision: https://secure.phabricator.com/D13142