use our own get_language_from_request instead of Django's

django's get_language_from_request has a global "_accepted" cache which,
for some reason not quite clear to me, always contains en_US. This interferes
with our allowed frontend languages: translation.get_language_from_request
returns "en" even if it's not an available language
Merge pull request #1719 from piquadrat/feature/frontend-lang-fix

use our own get_language_from_request instead of Django's