All right, so you bought the game from the Switzerland Mac App Store, but the language was in English. Hmm. I'd figure it would be in German, but googling this I see it's pretty confusing with the Switzerland Mac App Store. I have no idea how you would—or if you could—obtain a French version of the game from the Switzerland Mac App Store, sorry.
I'm also unsure where you could purchase a downloadable French copy of the game. But you can purchase a physical copy (DVD) with the French version on it from Amazon France:
http://www.amazon.fr/Amanita-Design-Machinarium/dp/B003FST9GK/ref=sr_1_2?ie=UTF8&qid=1363368956&sr=8-2Amazon France claims they ship to "any address in the world, except for Cuba, Iran, Iraq, North Korea, Sudan, Syria, and areas not served by the French postal service."
Or perhaps a physical copy of the French version of the game could be purchased from Ebay France? You might want to check that out as well.
Also, just to note that I'm not an employee of Amanita Design, I'm just a fan. :)
One last thing is to reiterate what Martijn has said, that only the menu items (i.e., save, load, etc.) and the text in the junkyard screen contain words that require translation—there is essentially no spoken dialogue or text in the rest of the game.
