Believe it or not the RAF has a linguist trade, although they tend to be a bit of a joint service asset, and an RAF JT (or whatever rank) linguist could easily find themselves on an infantry foot patrol as a ship or RAF unit. The way it was a couple of years ago was if you can speak a second language, go to the education centre and get the books full of military vocab in your chosen language. When you're ready book yourself into Beaconsfield for an exam at the appropriate level as an external candidate. If you pass you get a one off payment based on which list (based on difficulty) your language is in, and what level, then it gets annotated on JPA, and the RAF can call on your language skills as required.
The only people who get language courses are those that require them, either due to something like long look or a NATO posting, or RAFLO post etc.