Rated 5 out of 5 stars

Perfect. I came from Dictionary Switcher (https://addons.mozilla.org/en-US/firefox/addon/3414/) to this one.

The only (small) thing I am missing here (but this may only be a matter of taste): make locales switchable by clicking a simple left click. To make clear what i mean:
right click = context menu, showing all available locales.
left click = switch to the next locale in the list.

However: good one. 5 stars.

This review is for a previous version of the add-on (1.7.2).