Translated welcome emails 1029071

Kinda tricky to spot-check, but here it goes.
* [x] Extract new l10n strings: 
  * `./manage.py extract`
* [x] [Pseudo-translate](http://groovecoder.com/2011/11/18/pseudo-translation-to-test-i18n/) the strings into a single locale (e.g., French): 
  * `podebug --rewrite=bracket locale/templates/LC_messages/messages.po locale/fr/LC_MESSAGES/messages.po`
* [ ] Register a new account in [en-US](https://developer-local.allizom.org/en-US/)
  * You should see the welcome email output in the `foreman start` console output
* [ ] Register a new account in [fr](https://developer-local.allizom.org/fr/)
  * You should see the pseudo-translated welcome email output in the `foreman start` console output
* [ ] Register a new account in [de](https://developer-local.allizom.org/de/)
  * You should **not** see an email output in the `foreman start` console output
Seems like this could be:

    if (request.locale == settings.WIKI_DEFAULT_LANGUAGE or
        strings_are_translated(WELCOME_EMAIL_STRINGS, request.locale)):