Is it offensive when people from other countries do things that are definitely offensive in America?

I’m American and I moved to Germany a few months ago and have realized some things. First of all, so many white teens say the N word freely here. I was immediately shocked and angry and told them they shouldn’t say it because it’s a derogatory term that came from slavery times and is still used in a racist manner today. They just say they have black friends and it’s not mean. I don’t think they understand the whole American history with it. But am I wrong to think it’s wrong even if a German says it? Even if they don’t come from America? Also my German friend did a war cry and I got mad and they were just like” i played Indians all the time as a child what’s wrong with this” and also someone literally dressed up as a Native American to a Halloween party and I said it’s offensive and my friend said “it was your people who killed them, so when a German dresses up like them they are just having fun” Um haha what! European colonies slaughtered native Americans ! Am I being too sensitive and should I be understanding that they don’t think this way here and they don’t know better? Or should I tell them that things are offensive?