Google launched translation bots for a lot of language pairs, even more than the ones available at Google Translate. To use them in Google Talk or in any other IM client that supports Jabber, you need to add one of the bots as a friend, start a conversation with the bot and enter the text you want to translate. Google's bots have a simple format for addresses:
[original_language]2[destination_language]@bot.talk.google.com
For example, if you want to translate text from English to French, you need to add en2fr@bot.talk.google.com. To translate Chinese text in English, add zh2en@bot.talk.google.com. Here's the list of languages pairs and you'll notice many new pairs, displayed in bold below (some of the language pairs were subsequently added in June 2008):
ar2en, en2ar (Arabic<->English)
bg2en, en2bg (Bulgarian<->English)
cs2en, en2cs (Czech <-> English)
da2en, en2da (Danish<->English)
de2en, en2de (German<->English)
de2fr, fr2de (German<->French)
el2en, en2el (Greek<->English)
es2en, en2es (Spanish<->English)
fi2en, en2fi (Finnish<->English)
fr2en, en2fr (French<->English)
hi2en, en2hi (Hindi<->English)
hr2en, en2hr (Croatian<->English)
it2en, en2it (Italian<->English)
ja2en, en2ja (Japanese<->English)
ko2en, en2ko (Korean<->English)
nl2en, en2nl (Dutch<->English)
no2en, en2no (Norwegian<->English)
pl2en, en2pl (Polish<->English)
pt2en, en2pt (Portuguese<->English)
ro2en,en2ro (Romanian<->English)
ru2en, en2ru (Russian<->English)
sv2en, en2sv (Swedish<->English)
zh2en, en2zh (Chinese<->English)
zh-hant2en, en2zh-hant (Traditional Chinese<->English)
zh-hant2zh, zh2zh-hant (Traditional Chinese<->Simplified Chinese)
While this is a great interface for translating short texts (you can access Google Talk from Gmail, from google.com/talk or in many other ways), you can also use the bots to translate conversations in real-time. You need to invite the two corresponding bots in a group chat, so this doesn't work in the desktop client, which still does not support group chats. For some reason, when I invited one of the bots, it was offline, but it still translated my messages.
It would be great if Google adds this as a standard feature of Google Talk and you only need to enter your native language, but the translation quality is not that great and it could cause problems in some situations. The good news is that Google's statistical translation system advances really quickly and you'll see more and more languages pairs in the future.