Change and save language user specific

[Fasse]

Actually we can only set the language for the whole organization. It's not possible the change this per user. We should add an option for user specific preferences and than the language could also be saved user specific.

If a user is not logged in we should set the language from the browser settings. Only if that failed we use the language from the system.

The following sequence should be used the select the language:

external user:

1. use browser language
2. use language from organization preference

registered user

1. use language from user preference
2. use browser language
3. use language from organization preference