I've often said that when you learn a language, you also learn the culture which uses it - they mirror one another. Also, language is not something static, if you compare the slang and colloquialisms of different generations they vary a great deal. One of my parents' delights in their grandchildren is learning the idioms and slang of that generation from them.
The humour of another culture can be one of the biggest challenges of learning its language. One thing I love about the British is our ability to laugh at ourselves. Check out this sketch which is as valid today as it was when it was first produced:- https://www.facebook.com/BritishComedy/videos/1977077542509541/
Another thing to consider when learning another language is the distraction of regional accents and expressions - which can even be a problem for those who speak (more or less) the same language. In my first year at university, one of my flatmates was a lovely Irish girl who wasn't familiar with my then favourite brand of tea. One day, having put the kettle on, she cheerfully asked me if I'd like some of my old grey tea? I found it hilarious and guffawed for some time which baffled her somewhat. Eventually, I was able to explain that it was called Earl Grey . . .
If you'd like to join a small group which exists to give people the chance to interact in English in authentic situations or you'd like to follow more posts related to the challenge of learning British English, join the group I set up SLC Mark II (which is free - the only condition is that each member should organise an activity at least once every 3 months. Most typically this has been an afternoon picnic in the park):- https://www.facebook.com/groups/195214821015135/ I've often been asked the reason for the name which I freely admit is ridiculous! I made it up in a hurry so here's the competition and prize:- 3 separate hours conversation class to the person who comes up with the best new name for this group!