HC and Roundwego sum it up nicely, though I think the comment about "being God" is a little misleading. It may be that the average visitor doesn't get to meet the development test pilots, but if you do get the opportunity you would be hard pressed to find a nicer bunch of blokes - none more so that Didier Guerin, head of flight test.
HC - I think most of the production test pilots have done the 3 month short course at Ystres (EPNER.)
In addition to pilot positions, there are of course ground instructors with ETS - Eurocopter Training Services - who are the guys that run the groundschool elements of the Type Rating Training Courses and are generally ex-military pilots. Similarly, there are simulator instructors with Helisim which, although not technically Eurocopter, as a 45% JV partner is to all intents and purposes the same.
Frankly, speaking French is unlikely to help much if you are not, in fact, French, as far as working in Marignane goes. Elsewhere in the world I think the chances are much higher. In fact I was informally offered a job with them in 2001, but declined to pursue it as it was bad timing. The impression was that it would have been Far East based.