Some Swiss also speak Romansh.
I learned to speak German at age 35. Total immersion is the only way. Went to Germany and didn’t speak English for 5 months. Keep up with it by reading; especially dialogue so plays are a favorite.
Oriental business friends never get our colloquialism at all. You have to be careful and avoid our normal speaking patterns.
Even Germans don’t always follow our casual conversations, and they’re very cood.
That’s how people learn Hebrew they go to Israel and only speak Hebrew for several months
BTW When I was learning Hebrew (but here in the US) I found the form of writing very easy to learn...
I had learnt to write Pitman’s Shorthand in high school and they look so similar in part that I had little trouble with Hebrew...
My Dad missed out on a lot of American humor, and colloquialism could be challenging. As he hit his late 90's and had cognitive problems we noticed that his English deteriorated faster than the European languages he'd learned in his youth.