Nice, but there is a better one (Quick Locale Switcher) Rated 4 out of 5 stars

If you don't need the (IMHO far too slow) "detect as I type" feature, have a look at "Quick Locale Switcher":
I used Dictionary Switcher a long time and it works quite well (therefore 4 stars), but I'd like a way to select locales to show in the list (the list is very long on many Unix-like systems cause they often provide many locales). Additionally, Quick Locale Switcher provides some additional features beside the full control of the listed locales.

This review is for a previous version of the add-on (