To be a teacher of any sort you will follow the same path, for the most part. Teaching a foreign language is just like teaching any other school subject, the difference being that instead of being knowledgeable in Calculus, for example, you will have to be knowledgeable about French or Spanish or whatever language you choose. Requirements vary from state to state and school district to school district and maybe even from school to school. Some will require a bachelor's degree in the field you want to teach with a teaching certificate other will require a master's degree in the field but not a teaching certificate. It will also depend on the level at which you wish to teach. For example, college professors in general will require a PhD.