ICAO rules allow for various languages to be used but I believe English is preferred, I can't remember the exact phrasing. In the UK we obviously use English and all pilots must have a level 5 or 6 English proficiency. Good job for me, I think I would have trouble telling the Indonesia 088 to hold at Timba (Gatwick hold) if he couldn't speak English.
The French/Spanish controllers usually speak to their country's pilots in their native language which I believe makes it difficult for none French/Spanish speakers to build up a mental picture of what is going on. A fatality occurred a few years ago at CDG with an MD80 and Shorts 330 colliding on the runway, language differences were a part cause I believe.
If the only aircraft on the frequency are native speakers then the controllers will use that.