Crossing the boundary of standard phraseology into Courtesy in RT is... tolerated pretty much everywhere. How far can you go it is (as I understand in your case) sometimes defined by your sup/competence officer.
Lets say that courtesy in english it's the accepted "norm".
From here on...
In another bi-lingual country they mark the paperstrips of the french speaking pilots so they can use french with them and english to others. Greetings follow the same rule, french with the french speaking and english with the rest of the world. But even this is not that obvious in this globalised world when the pilot of a german company can have a noticeable french accent.
One of my colleagues was driving me nuts by greeting everybody in THEIR native language. When having a more "exotic" company I would hold my breath to see whether he "knows" the greeting in that particular language. No harm done, even amusing at times. For the record, he was "suggested"
to stick to english, so it did not happen only to you in this world.
On the other hand It is corny when pilots greet me using the language of another country by ignorance (I don't mean english here). It is one reason I agree with my colleague being told to stick to english.