Hi Dhuberry,
Are you sure that you are providing the correctly encoded data to the streaming API? The post data for this example should be :
track=Th%C3%A9+noir
And if you want to further help ensure French only, use :
track=Th%C3%A9+noir&language=fr
The search API does normalise characters such that ‘é’ would match ‘e’, but the streaming API matching is more specific, so you should only be getting tweets that contain exactly those two words (though potentially in any position in the tweet).
HTH!
JB.