So if it doesn't list English in the top three native language guesses, does that mean my English is very ungrammatical, or merely that it's ungrammatical in a particular way? All three languages were Scandinavian but I have no connection there.

The NY Times had a similar quiz that attempted to predict your region and, IIRC, predicted mine fairly closely. I didn't get such accurate results this time so I'm wondering if my grammar is slipping ...