Detect language from browser (if not logged in)

[wq9578]

It would be desirable to have the language detected from the browser setting, if not logged in, as done here.

If logged in, the user locale should be stored in the database, as somebody already suggested earlier.

[bezhanSalleh]

It already does, if the user locale is one of the locales you have enabled for the switch.

[wq9578]

There is a bug, so it doesn't work in most cases: #68