I just tried it with French (a language I already know).. and I don't think that it's a good tool. In my opinion, you should always avoid seeing incorrect usage of language when learning. This extension does not properly translate the words in context, it does not provide the proper forms of words that correspond with the subjects/objects (such as masculine/feminine/plural), and sometimes it's completely wrong. If you learned using this tool, you'd have to unlearn almost as much material as you learned.

I think it's much, much more effective to just read a newspaper in the language you want to learn and use a dictionary. It's not as easy, but you're not going to expose yourself to bad grammar that you'll automatically pick up and have to unlearn later.