Currently Psi supports that the messages you type in, are checked against a spellchecker.
But only for the from the system defined language, even the spellchecker would support more then that language.
So there is a need (IMHO) for a selection while typing the message, cause the most user of Psi will min. speak two languages (english and the mothertounge).
Here is a alpha implementation of this, it will let you choose the language in the chatdlg. It’s far away from being tested overall, rather than it worked for me in the first view. It is worked out against current devel version of Psi (0.13-dev) from git.
Here is the patch:
multiple_spellchecker_languages_support.tar.gz
And here a image how it should look like if it has compiled successfully, of course with the languages you have installed on your system 
example_multi_lang_spellchecker_lnx.png
The current implementation works only with aspell, which is used on windows and linux. But windows I never tried . Build and quick try, worked for me:
example_multi_lang_spellchecker_win.png
So mac os x (10.5 or later only) is now supported too.
example_multi_lang_spellchecker_mac.png
And the patch which includes everything about multi language support for Psi spellchecker is here:
0001-overall-multiple-language-support-for-spellchecker.patch.zip
- the patch only includes ui support for the chatdlg, muc I haven’t done yet.
- the language combo box needs to recognize language changes / or on startup too,
so it displays always the correct current language.
Ciao Ephraim