Why do some people think they have to change who they are and talk a certain way just because they are talking to people of a different race?

A serious reply. In over 50 years of much international travel and working in different cultures, I have never once seen an example of what you describe when it is done without the complicity of the interlocuter. So that when groups of mixed cultures and ethnicity get together, they willing and in good humour and friendship share cultural and linguist differences. Often it is self-mocking! Because we all know the idiosyncrasies of our own tribe, our own nationality, our own race and so on. Sophisticated and intelligent people are the finest mockers of their own status.