Tested with a fresh new profile on Nightly (33.0a1 (2014-07-16)) on GNU/Linux. STR: - install French dictionary from https://addons.mozilla.org/en-US/firefox/addon/dictionnaires-fran%C3%A7ais/ ; - go to any webpage with a textarea ; - right click inside the textarea and switch to Languages / French ; - repeat ad lib : nothing happens, the spell checker stays in english ; - cry, because now you fear everything you type will contains mistakes.

Also confirmed on Win by some colleagues of mine. This seems to be a recent regressions, it used to work fine until yesterday.