Ok, so previously translation of "kolega" was "a mate" o "a colleague". Now I translate this sentence and right translation is "This is my friend from work" instead of "This is my mate/colleague from work". Any explanation?
Well, I think that's mostly because English uses "friend" a lot more often than Polish uses "przyjaciel". "kolega" doesn't have to be anyone close, but often English would go with 'friend' anyway.
"colleague from work" works, "mate" is too colloquial, I'd say.
I AGREE: 'CO-WORKER' IS USED IN THE UNITED STATES, RARELY IN BRITISH ENGLISH. 'WORK COLLEAGUE' OR 'COLLEAGUE FROM WORK' WOULD BE MORE USUAL.