Script Summary: Adds an "translate" icon to new Twitter tweets, allowing you to translate them with Google's translate service.
If you use Chrome, you should be able to just click on the install link straight away. Chrome supports Greasemonkey scripts out of the box.
If you use Firefox, install Greasemonkey first. Restart the browser, then click on the green install link above.
If you use Safari, you could try NinjaKit, but that would probably involve some translation work right from the start – it's in Japanese. Instead, you might want to try Josh's TwitterTranslate Safari 5 extension. It works slightly differently, but the end result is the same.
Okay, so now restart your browser, log into Twitter, and mouse over any foreign language tweet. You should see a new action below the tweet, next to favorite, retweet, and reply: translate.
Just click on the action, and in a few seconds, Google Translate will automatically replace the tweet with one in your own language.
Let me know if it doesn't work out for you.