In Japanese, when people want to say “never mind”, they say,
It is interesting that some people say,
I believe that it originated from the English phrase “don’t mind” and was modified into Japanese English which means “never mind”.
You hear “don mai” more often by people playing sports.
If you want to say it more politely, you can say,
Kini shinaide kudasai.
Japanese people often add,
I am fine. / No problem.