Fascinating. I agree so much about the inseparable natures of language and culture.
I speak English as a native, and learned French first as an adult in Montreal. Later, I spent years immersing myself in literary French and metropolitan French.
Sometimes, when sitting at a table full of Québécois speakers and French speakers from France, my brain bounces around like a ping-pong ball. The languages are perfectly mutually comprehensible (among educated speakers anyway) but the cultural references and connotations are very distinct.
Once, at a birthday party that included many monolingual American English speakers and a couple monolingual French speakers, I volunteered to interpret all the toasts.
Bouncing back and forth between the two kinds of French exhausted me because I kept having to work so hard to switch cultural contexts.
Going from English to French was easier. But from French to English was quite tiring.
I don’t know the solution. I know both kinds of French too well to be neutral. When I code switch, each type of French is its own distinct page.