I think sticking to OPOL for every single little detail of your life is rather going against common sense? People all over the world sing songs in languages they don't understand, they don't become fluent in those languages as a result and neither I think does it do them any harm.
It is pretty amazing how quickly children can copy words when they are sung though. I work with a child who most definitely is on the autistic spectrum, he can sing far better than he can talk and pronounce sounds when sung better than when he says them without music. A new boy with no English started this week, 5 minutes after finishing a music session, he was the only one still singing and repeating, will be quite some time before he understands it of course!
Will be watching this with interest. I'm Swiss and speak German with DS, DH speaks English. I sing in both languages, maybe even more in English because I'm struggling to remember the German ones I'm thinking, as long as you sing anything, hopefully it doesn't matter which language...
my dh speaks French to our lo who is almost 5 months old whereas I speak English to her.
we both like to sing to her and have done this since she was born. but do we need to stick to one parent one language when singing as well? I find some of the French songs better for soothing her. will she get confused if the French parent sings in English and vice versa?