fix bug 1029071 - translated welcome emails

Repeat instructions from https://github.com/mozilla/kuma/pull/2532#issue-37096580 ...
* [x] [Get the localizations](http://kuma.readthedocs.org/en/latest/localization.html#getting-the-localizations)
* [x] [Add a `welcome_email` switch](https://developer-local.allizom.org/admin/waffle/switch/add/) set to active
* [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.pot locale/fr/LC_MESSAGES/messages.po`
* [x] Compile the new `.mo` files for python
   * `./locale/compile-mo.sh ./locale`
* [x] Restart django server to load new translation strings
* [x] 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