Localize dates and times
Add missing translations
Fix KeyError when accessing User.displayname

After calling db.session.commit, User.__dict__ does
not contain the attributes anymore, until they are
accessed as properties again:

>>> user.__dict__["firstname"]
'Max'
>>> db.session.commit()
>>> user.__dict__["firstname"]
[...]
KeyError: 'firstname'
>>> user.firstname
'Max'
>>> user.__dict__["firstname"]
'Max'
Fix error when displaying form errors

WTForms seems to think it's a good idea for field errors to sometimes be
strings and sometimes dicts.