This useful Firefox add-on harnesses the power of Google's translation services to quickly translate the text on a web page. Just highlight the words of interest, right-click, and choose the translate option. If you've only chosen a word or short phrase then you can read the results in the context menu; anything longer and you'll be able to view the results at Google Translate, in a click.
The reliance on Google means gTranslate will only support the same 25 languages, which are currently English, Arabic, Bulgarian, Chinese, Chinese (Simplified), Chinese (Traditional), Croatian, Czech, Danish, Dutch, Finnish, French. German. Greek, Hindi, Italian, Japanese, Korean, Norwegian, Polish, Portuguese, Romanian, Russian, Spanish and Swedish. It's a good set, but if you need to translate Hebrew, or Thai, then you'll be better off visiting a website like Bing Translator (http://www.microsofttranslator.com)
For the most part gTranslate works very well, though. It can automatically identify the original language to save you time, is able to translate image titles and alt attributes, and can even work with online forms, replacing your words with the appropriate translation in a click.