DD has simply picked up enough Italian to read reasonably well even with some limited exposure to Spanish and Portuguese as a younger child. I think it's the classical musical training, myself.
She's studied German-- which I've personally found more useful than French as a scientist-- and plans to study French, which she picked up with relative ease even in two weeks in Paris.
She may not be most kids, though, and would like to study either in French-speaking Canada and/or in France at some point in her future.
Agree that French/Spanish/Italian/Portuguese are more or less interchangeable in terms of learning demand and ease of conversion.
Dutch and German share enough similarities that those two are similarly interchangeable.
My answer, in summary:
STEM? German.
Humanities? French.
(I say that having minimal competence in either one-- I've used them both, but in very different domains in my life.)
