Yes, a foreign legionnaire may apply for French citizenship from three years of service.
It is usually granted, subject to have a good way to serve and have proven its willingness to integrate the French Nation.
The legionnaire who does not wish to opt for French nationality retains his foreign nationality and is just as much to stay in France at the end of his contract and once he is in possession of a residence permit.
Obtaining a residence card is consecutive to obtaining a "certificate of good conduct" issued by the command of the Foreign Legion to each deserving legionnaire leaving active service.
Furthermore, the legionnaire wounded in operation may acquire, from right, French nationality (law called "the blood shed").