Children have a unique ability for language acquisition. I don't think you should worry, the only important advice I would give you is to be consistent when using different languages. Don't change from one language to the other in the middle
of the sentence, and use proper language use. If you're not proficient in a language, leave it to others who are to talk with the child.
As for teaching the child prayers... meh, I don't know. It can be troublesome when you yourself are an ex-Muslim and don't see the point. But try to explain to your in-laws that anything that is taught to a little child should be age appropriate and without force. I always wanted my daughter to become hafidh quran, but I never forced her to learn even the shorter sooras except if she herself showed interest. She loved listening to my recitation, but was stubborn like a mule refusing to even recite qul huwa allah ahad even though I knew she could