A location seems in place. It seems a lot of users from the US prefer to avoid any kind of conflict unless absolutely necessary. Whereas in The Netherlands it would be perfectly acceptable to laugh in their face and tell them they sound like a fucking retard, certainly if they do not report to you.

I agree that the answer may depend on the location. Certainly where I work, making fun of the way someone talks or using a phrase like "fcking rtard" would be considered rather adolescent and immature. This has little to do with conflict avoidance and everything to do with conducting oneself in an adult manner.

@eps So using words from a particular register of speech is “acting like an actual baby”? Of course it’s not. He’s acting like a perfectly normal adult, but choosing words that are unusual and excessively bowdlerised for the context they’re used in. However, something that is exactly how a baby (or a child, at any rate) would act is mocking others for slight nonconformities. That’s the kind of behaviour you’d expect from small children, but not from adults.

Maybe an anonymous and kind note?

"he is native to the US" - well, obviously. The only question is how close I can nail it down : a single, 45yo white dude, from Michigan? Maybe Wisconsin, eh? Ope, for Pete's sake, jeet? Well, you'd better get on the expressway before you get all snookered up, and tell you mom's I says hi, dontcha know.

Baby talk is raising the pitch of your voice and/or using gibberish. Don't make fun of people's dialects.