Change spellchecking language

On Ubuntu 11.10, the spellchecking dictionnary seems to be set to english. I want to change it to french.
I have tried to set the sppellchecker.dictionary preference (see http://forums.zotero.org/discussion/6393/spell-check-notes/), but it doesn't work. Any hints ?

(In Firefox, french spellchecking works)
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