Translation fixes

* Updated all translations from Transifex
* Updated translation source to Transifex.
* Fixed Arabic #915 
* Added Khmer translation (Had to fix some plurals) #912 

The latter was not working out of the box: For some reason, Django has translation for `km` while the locale system references only `km_kh` and not `km` alone (as it does with most locales).
However, if the user's Accept-Languages contains `km-kh`, it works as expected.
Because Firefox UI (not sure about other browsers) only offers `km`, I've added a Middleware which replaces the LocaleMiddleware from Django and replace `km` with `km-kh` if required.

Tested on dev.fh