[jajko]

Speaking (and thus cca thinking in) foreign languages massively stimulates your brain. You can't do much more for your (or child') mental development than make it learn and keep using multiple languages. Age doesn't matter, you arrive at the destination just a bit slower - Ie I am in my forties and learning french, slowly but surely.

If they spot you, 2 things may happen - they will be nice like all normal people anywhere do and maybe even appreciate that other people are learning their non-trivial and probably a bit obscure language.

Or you hit the rest, and they will either laugh at you, ignore you or just switch to english if they want/can. This is very common in France in my decade and a half experience of going there almost every weekend, also very frequent among young. Their own shame and mistake, instead of embracing the future and improving themselves, they choose the other direction and watch world slowly pass by. I think it comes from their deeply flawed education in this regard, not on languages per se but whole view on exceptionalism and what current world actually revolves around.