Google recently rolled out a 20% project that allows you to translate text using the google talk client. This works with both the desktop version and the gmail version.
You add a bot to your friends list that is specific for your language translation.

Example: fr2de@bot.talk.google.com.
translates the french that you type into german.
There are currently 24 bots: ar2en, de2en, de2fr, el2en, en2ar, en2de, en2el, en2es, en2fr, en2it, en2ja, en2ko, en2nl, en2ru, en2zh, es2en, fr2de, fr2en, it2en, ja2en, ko2en, nl2en, ru2en, zh2en

Happy translating!