I've already talked with my parents about it, I am 21 and I have no intention to grow older and still be afraid of the possibility that one day you may be detained. What is great about France is that it gives people that fresh start, that one opportunity that we seek in this country that we may not be able to get any time soon.
According to some of the research that I've done, it's possible to become a French citizen after three years in the Legion. Since you've done your research I suppose you know the dangers, and the possibilities of not making it through that five year contract. All I can recommend you to do is to talk seriously with your family, at the end you'll decide on what you want for your life.
I'm also Mexican.
The FFL site:
http://www.legion-recrute.com/en/