My experience as a non-commissioned member, an NCM, and also as an officer is that what we were often told was that even if you were a corporal but you wanted to get that French-language training, you were out of luck, you were S.O.L. Often, you had to be a master warrant officer or a chief warrant officer or, in the officer corps, a major.
There are many people who want to progress and/or are medically released, but if they're below that rank level, they're out of luck for French-language training.