UI: Change Language: use a dialog instead of forcing first-time-wizard

The method of forcing first-time-wizard wasn't very effective because
1. it's ugly, and 2. after selecting the language at the wizard it either
imports the previous settings (together with the previous language thus
overriding the new language) or resets all other settings - which is not fun.

This patch uses a small dialog instead to only change the language, and within
it reuses the language panel from the first time wizard.