But a French controller at CDG wouldn't try talking to a UK reg in French nor to a D reg in German, English would be the best option, even though ICAO Annex 10 Vol 11 para 5.2.1.2 stipulates French, with English on request.
What you are trying to balance here is an improvement in SA for aircraft on the same frequency against an increased risk of misunderstanding between local language speakers where in the event that an instruction to a particular aircraft needs to be given quickly and accurately it is better done in the local language rather than translated twice.
eg. an immediate instruction to prevent an accident or incident, while nice for others to understand, would be better for the crew to which it is addressed to be clear and unambiguous and in the language that both they and the controller understand best.