What should I do if, after choosing a Master Thesis topic, I find an existing thesis on it in another language? I picked a very unique topic for my Masters Thesis, only to find out that there is a Dissertation on the same topic, from 2008, in French. No other books have been published on this topic.

Ugh... I really really wanted to do this. What should I do? Can I do the same topic in English? I can't even translate what the other person wrote.
You can still write on the same topic, you just need to expand the knowledge that's already there and offer some new, original insight to the issue.