Seems that this is one of those words that can be translated many ways since it's an exclamation and it might depend on the circumstances. Exclamatory words run in a cycle of popularity - for example, one might even say "cool" presently
I suppose there's so many synonyms for something like this that is impossible to get them all. My polish GF overheard the audio just now and said it meant "fantastic" so that's what I entered and got it wrong.
I haven't checked if Duo accepts it, but I think "Brilliant!" is the most exact English equivalent of the Polish "Świetnie!" (cf. "swiatło" = "light").
EDIT: Duo currently accepts "brilliantly"in the English translation of "Jej mąż świetnie gotuje" (Her husband cooks brilliantly).
why isn't "marvelous" an acceptable translation. Translation can't always be literal.