R1:e84f9f9ec99c

Send Differential e-mails in user's language

Summary:
Works this way:

- Select users' language with multiplexing.
- Select default language otherwise (it can be different from current user's language).
- Build body and subject for each user individually.
- Set the original language after sending the mails.

Test Plan:
- Comment on a diff of user with custom translation.
- Set default to a custom translation. Comment on a diff of user with default translation.
- Set default to a default translation.