I know it's a bit late to talk about it...
Every year in Paris, there is the biggest french japan convention.
http://www.japan-expo.com/en/This year, it's from 1st to 4th July. There are cosplay contests, music, many stands where you can find anything about video games, anime or mangas. There is also a part where people sell mangas they made themselves. Some of them are very good.
There is also the "Chibi Japan Expo", which is the same, but a bit smaller. It is usually end of october.