The "rules" would probably depend upon which country/airspace you're talking about.
In Canada, for example, it's not uncommon to hear French spoken on frequency or even to have a different frequency for an ATIS in English rather than French.
I suppose the only guideline that is (hopefully) universal is to consider all parties that might be interested in a given transmission. For example, if ATC is issuing a traffic alert to two aircraft that are about to be in conflict with each other, and if that controller knows that Pilot A only speaks English but Pilot B speaks English and Romanian, it would probably be wise to issue the entire traffic alert (addressing both aircraft separately but in the same transmission) in English. Later, the controller may decide to switch back to speaking with Pilot B in Romanian once there's no potential benefit of any English-only speaking pilots understanding the transmission.
The only area in Canada, if I'm not mistaken, where ATC communication are available in both English and French are regions inside the Montreal FIR.