launch khmer, update language

@rdbartlett, this will need some testing - specifically Khmer which is a new language.
please also contact:
  - the Khmer translators
  - the Belerusian translators (to explain that Belerusian is now a hidden language option and how they can try it)
 
here's a command you'll need when translations change for the frontpage. (maybe talk with rob about whether it still needs caching)

**dump frontpage cache**: 
```
heroku run --app loomio-production rails runner -e production Rails.cache.clear
```