This reverts merge request !1526

This reverts merge request !1494

This seems more intuitive to me and is what I've seen in most other
language pickers.

Reduces the size of the initial app bundle by about half.

better categorize it / adjustments to language selector

The locale can now be configured in settings and is stored in
Vuex. The changes are applied immidiately after selection. The list of
languages is taken from the messages file, which contains all the
available locales (and a new value, `interfaceLanguage`, to control
the translation of this option in the options menu)

Closes #36