Don't force the use of Accept-Language anymore

This is not intended anymore, since it falls back to force english
when the header is not set. Also 0228bc6e
makes clear that the order should be:

1. User setting
2. Accept language
3. Admin default

This is the case since the commit from above, unless via OCS and DAV.
Both forced to accept-language falling back to english.
By removing the force, it now also matches the w3 priority list:
https://www.w3.org/International/questions/qa-lang-priorities



Signed-off-by: Joas Schilling <coding@schilljs.com>