This on occasion can actually be dangerous. By saying "good morning" or whatever in the local language in a busy ATC environment can be grasped by them that you understand the language and chaos can result.
Before I retired, my last company used to send us down to Spain very frequently. We had a Spanish associate company and, in order to meet the 5th freedom rights/who pays the navigation bill/etc we had to use the Spanish callsign.
This almost inevitably resulted in a torrent of instructions in Spanish! On one occasion I landed in Madrid and could not get through to the sexy (young?) lady on the Ground frequency that she was going to get nowhere with me unless she spoke English. I finally resolved the situation by pointing out to her that I was a "Gringo"!
I would say that it is perfectly in order to say goodnight in the local language after you have set the parking brake.
As salaam alleykum.
PS. Can any of you out there remember Dan Air teaching the expression "Chugalug" to ATC all over Europe. I suppose in these days it would be considered definitely "non-PC". The odd Maastricht controller still comes out with it.