Technically, all ATC and pilots MUST be able to speak English. Realistically, ATC has to know English and pilots flying out of the country need to know English. But in most countries, the native language is just fine within the borders of said country.
In Indonesia, all ATC conversations are carried in English. However, sometimes you hear the local lingo when Ground is talking to airport vehicles. Some small airfields talk in Indonesian.... but I'll let the regular flyers here tell me instead of me telling you...
When losing situational awareness, pray Cumulus Granitus isn't nearby !