Allow setting instance of PhutilTranslator

Summary:
I will need it for sending e-mails. The plan is this:

• Set instance to recipient's language (or server default if multiplexing is disabled).
• Build e-mails.
• Set it back.

The other option would be to give up on using pht() and inject $translator everywhere and use it instead but it seems too complex even if it would be cleaner.

Test Plan: New test.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2772