i once heard that the French foregin legion was based manly on non-french who went in jail,is that true?
(This reply is not directed necessarily at you, just a general comment to the OP).
It used to be. Not any more.
The Legion is open to all (including French Citizens), but you will temporary renounce your citizenship for the first 2 years. After which you can choose to regain your identity, or you can stay no-identity and become a French Citizen and the end of your service.
As for a criminal record that depends. The Legion will overlook minor offenses, and
certain more serious ones. However Drug-related, and Violent Criminals need not apply, neither should anybody who is a fugitive because they will hand you over directly to Interpol. The days of the FFL being a safe haven for criminals is over.
Whats most important during the interview process (and I cannot stress this enough), is not to lie during the interview. The FFL uses a very strict honor system, violating that system brings out some very sever penalties, penalties like corporal punishment. If you are hiding something about your past they will find out about it (they do check) and then your as* will be toast.
Be Warned: the FFL is extremely tough, it does not have the same rules+regulations as regular French Army does, the lifestyle is very Spartan by comparison. My father-in-law (hes a Colonel) says most of the FFL recruits had prior military training, although this is not required. Be prepared to stationed a good length of time in some world pit. Its not fun and games.
I have not been in the military myself, so I will not give you any advice about what you should do. I have relatives who are officers in the French military, if people have further questions ask or PM me, I can check it for you. Anything I cannot answer, I will say so and direct you to the FFL Recruitment website.