[Feature Request] Show a dialog when user has change the language setting

[gunchleoc]

When the user has changed the language setting, it won't take effect until the program is restarted. There should be a dialog box informing the user of this, something like "Restart UltraStar Deluxe to apply the new language setting."

[HermannDppes]

As language changes go into effect immediately now, I guess this can be closed.