Shoe on the other foot...
English is my mother tongue, which was a big help when I went into aviation. I only noticed the disadvantages to being mono-lingual once I started working outside the U.S.A.
To fly GA aircraft in Germany I had to learn some basic aviation German and pass a language test. This was the first time that very basic point that English is not, in fact, always and everywhere the official language of aviation was brought home to me. Well, yes, there had been the odd occasion when a trip to Gabon, say, saw most of the traffic speaking French but I took that for an aberration.
If you want to fly at an airstrip in Germany AND use the radio for getting advisories and giving position reports then you shall need a specific radio licence that states you can speak German. (You get into a situation where the others aren't allowed to speak English, having German-only licences, while you aren't allowed to speak German, even assuming that you are capable of that, if you lack a licence for that.)
Now I am working in North Africa, where the two main languages are Arabic and French, with French sharing about 50% of the radio traffic with English at a rough estimate.
For your own sake you had better know the French for the basic clearances so that you have good SA. To get fuel you shall need very basic French to give the quantity required unless you want to be reduced to writing little notes on scraps of paper, when you may look like an ignoramus, or else get the other crew member to do this for you.
Most of my colleagues have English as their second or third language. Socialising is done mostly in French and Arabic with English in third place closely followed by German. You wouldn't get very far here by putting in some sort of "English only" rule anywhere but in the cockpit, and even there English is restricted in its use if both crewmembers share another common language. I think this is just common sense, really.
Such a heavy-handed approach as ordering people to only use English just because they are working in aviation would be totally unacceptable, I think. It would be interesting to see someone try to do that of course. "Cultural imperialism" might be the first bit of English learned aside from the usual stuff such as "Say again?"
Having the courtesy to learn some basic phrases in the local language can pay big dividends in having a good working relationship plus helping to make friends if you have any interest in that. (411A, you can just look away now!)