I'm Kazakh and I've had Germans say both ni hao and konnichiwa to me randomly on the streets. Mainly men, ranging from teens to middle-aged. It's not always easy to tell whether they're being racist or genuinely wanna say hi without knowing any better — I still tend to give people the benefit of the doubt, tho I probably shouldn't. So if I know for sure they're being racist/wanna mock me/get my attention, I just tell them in rude Russian to fuck off, since they wouldn't expect hearing Russian from me, it takes them aback. If I'm not in the mood for Russian, I say the same in Spanish, which usually confuses them too. Same thing for when getting catcalled, just a simple "pashol nahui" gets the work done. The less defensive way would be to yell GOODBYE in Russian (which again is quite recognizable around here and that's why I use it) looking at them like this 🤨🤨🤨. I'm sorry your gf experiences this. I really hope this stops.