Add L10n factory method for generic language heuristics

The existing `findLanguage` method tries its best to find the best
language for the current users. For some tasks we don't want this but
rather determine the most generic language for *another* user, e.g. when
the current user trigger an email notifiaction to someone else. In this
case the current user's language is a bad guess in many multi-language
environments.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>