if in doubt, be polite.
The person in question WAS being polite. He was speaking HIS native language in its own historical and linguistic context, and using a perfectly appropriate word. How could he have known all the possible associations of the word as used in other countries? For the Polish speaker, a "mulat" is simply someone of mixed black and white parentage: sjp.pl/mulat
(as you can see, the Dictionary of Polish Language does not even list any slangy or derogatory meanings of this word, as it is too high-brow in Polish to have any).
Wiki says this on the subject: en.wikipedia.org/wiki/Mulatto
A relevant quote:
"Contemporary usage of the term is generally confined to situations in which the term is considered relevant in a historical context, as now most people of mixed white and black ancestry rarely choose to self-identify as mulatto.
The term is generally considered archaic by some and inadvertently derogatory, especially in the African-American community. The term is widely used in Latin America and Caribbean usually without suggesting any insult.
Accepted modern terms in the United States include "multiracial", "mixed" and "biracial". (Highlighting mine).
1) A large part of the world (Latin America and the Caribbean) seem to have no issues with "Mulatto", from where this term was probably borrowed into Polish in the first place;
2) Polish people do not live in the United States, and do not speak English, but Polish;
3) For Polish speakers, terms such as "multiracial", "mixed", or "biracial" smack of Nazi terminology, with which we have had the occasion to acquaint ourselves in practice; I wonder whether English speakers ever think of this when they use words like that in Polish? In other words, are they in doubt and do they choose to be polite? I doubt it.