Possibly this one will be compatible sometime in the future:
Autoswitch isn't working at the moment (mostly fixed, but still having some issues with multi process Firefox).
Hopefully the same as Quick Locale Switcher, but less buggy.
This is a fork of QLS, since it seems to be quite dead (developer wise).
If you wish to have the flag show up in the statusbar you'll need to install
Otherwise there is a toolbar button, or a menu in the Tools menu.