In Patricia Wrede's Mairelon books, the important thing was to NOT use your native tongue. There was something about all the nuances of your native tongue that made spells give responses that were inconsistent. Which produced the amusing side-effect of magicians coming from foreign lands to learn English to do their spell-casting.IIRC, modern theories regarding learning language have it that there is one center in your brain for your native tongue, and one for all the other languages you have learned. This could tie that together nicely.
I've not encountered those books, but that sounds quite fun :-)