My wife and I are in the trying-to-conceive phase of having children, so this may be just me being baby-crazy and overthinking things, but here goes...
We are both native Anglophones, but for different reasons have taught ourselves French - to a level that we regularly use French at home. We intend to raise our children speaking French, and my wife supports a mLaH approach. While we continue living in California, I feel confident that our children will also learn fluent English from the environment (especially if we teach it as a formal subject). So far, so good.
My wife is Jewish, and teaching herself Modern Hebrew - another language she wants our children to learn [beyond the Hebrew school standard for b'nei mitzvot that ends up forgotten by college]. I've been hacking at it and can recognize a few phrases and the aleph bet. We are intending to make aliyah and, at some point in the next few years, and move to Israel. We would be in Israel long enough for my wife to study for her rabbinical degree, if not longer. Our family would be surrounded by a rich and dizzying language environment (including English and French), but one dominated by Hebrew. And if we only speak French at home, I would seriously start to worry about our children's ability to speak English to their grandparents.
My question for multilingual families is - how have you handled the move to a completely different language environment? Have your dcs atrophied in one or more of their languages? Is it an overblown fear? How have you worked to save-guard the newly-minted "minority" language(s)?