My sister, the niece's mother, is mostly concerned about the child's delay in producing speech in either language. The kid definitely understands pretty much everything that is said to her in Polish or in English, but she still only speaks in single words. I'm mostly concerned about the apparent mixing of languages.
Please, tell me that I'm over-reacting and, please, also give me some way to assuage my sister's fears. There isn't as much advice on raising multilingual kids out on the Internet as I would've expected.
P.S. My niece hears English all day at daycare (~3-4 days a week) as well as on television (including stuff like Sesame Street; hence, the counting and alphabet). Her parents speak to her in Polish. (And, yes, my sister's been teaching her to count and read/spell some words in Polish.)
EDIT: Thank you, everyone! I feel much better now.