Language setting

[blutorange]

Low priority.

The user should also be allowed to change their preferred language. Should probably go to the settings tab of the profile.

This should probably be saved in the database as a setting for each user.

When getting the language, the order might be as follows:

• Get language from URL param lang, if it exists and save it in the session.
• Otherwise, get it from the user's settings, if it exists.
• Otherwise, get it from the session, if it exists.
• Otherwise, get it from the HTTP request, if it exists, and save it in the session.
• Otherwise, default to 'de' and save it in the session.

[blutorange]

This would also make for a not-so-large UML diagram.