Thanks for your info @Neo4316! I think my Windows language settings are OK. I will check again, also text to speech settings, speech recognition,… if I observe again a wrong ATC call sign with Real LiveTraffic I will try to document more (Flightnr, ICAO airline, destination, and make some screenshots,…) and make a Zendesk ticket!
Happy Flying!
EDIT: Probably it was because I used the ATC European Phraseology 1.4. MOD. I did some more tests and switched to the standard ATC file. Several hours of observing ATC traffic at EDDF seems everything is normal now.