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Check out Evan Mcgormans book about the Legion its good. Next I would say you need to do chinups running, hiking, push ups sit ups and optionally swimming(if you can't swim they'll teach you.) You do not need to learn french before hand they'll teach you. Seriously read that book though its the best book ive found
Assclownery & squawk7700 gave you some good links.
I as well was considering joining the FFL back in the day. I was a member of cervens.net for a few months & asked many questions.
I remember 2 things I was told: learn to be hungry a lot, & learn some French.
"Life in the French Foreign Legion" is available on amazon.com:
I own it. I think it's an incredible read. There's also a book on the history of their Paratrooper Regiment.
Mr. McGorman once answered an email I sent him & provided me few links.
After all was said & done I decided to join the U.S. Army (19D- Cavalry Scout). I emailed him after I signed up & gave him a hearty thanks for his efforts. He emailed back a few days later telling me I picked an excellent MOS & wished me luck in Iraq/Afghanistan.
Hope this helps.
You'd have to be partially insane to go this route... read this first https://smile.amazon.com/Life-French-Foreign-Legion-Expect/dp/1555716334/ref=sr_1_2?s=books&ie=UTF8&qid=1476741304&sr=1-2&keywords=french+foreign+legion