Projects - add translation for watcher strings

Summary: Fixes T7319. ...except I can't get this working in my sandbox? Changes to the translation file don't seem to show up. TEST PLAN IS A LIE

Test Plan: became a watcher, un became a watcher - saw sensical translated strings

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7319

Differential Revision: https://secure.phabricator.com/D11812