It's 2012, and i instaled recent MODX Revo with Babel and FormIt. Simple steps to translate to your language:
All I had to do is duplicate core/formit/lexicon/en
and changed it to core/formit/lexicon/pl
In FormIt snippet i didn't change anything. Don't have to use &language parametr.
I make a translation in Lexicon in Modx manager in Formit with new created lang PL.
Refresh your cache, refresh your browser - and thats all!
You have multilingual site with translated Formit erros. (Just make sure your Babel is correctly configured)