Cleanup: Remove unused import
Removing Languages from Django admin

The preferred way to alter languages is using our ADMIN. Also moved
the code to forms.py and did minor simplifications to form code.
Bugfix: Make delete action work again

Before this the link used in the delete action was not providing a
correct URL (without the language code at the begining) and thus the
action din't work.
tests: Drop a flaky test
tests: Mark two broken tests as xfail
tests: Unmark a couple of working tests