Update arcanist to work with more modular translations

Summary:
Ref T7152. Ref T1139.

• Tweak API.
• Move translations out of init file.

Test Plan:

• Ran arc.
• Added a goofy translation and made sure it was working.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7152, T1139

Differential Revision: https://secure.phabricator.com/D11746