Microwave is called “Denshi Renji” in Japanese. “Denshi” is an electron in Japanese, and “Renji” is maybe a Range of English.
A Japanese who can not speak English does not understand what “microwave” is. Of course they can not understand “heat up” or “warm up”.
So when you buy something at the convinience store and you want warm it up, say “Atatamete kudasai” to the clerk. Or simply say “Chin”. “Chin” is the bell sound when the time up of the microwave. You can order by the short word.