Not unlike Canadians with French. Now I speak French most of the time and I made the mistake of telling a woman to speak in whichever language she preferred. She didn't speak French, she spoke Quebecois. They may think it's French but it's no more French than Cajun. I've never made that mistake again.
Thing is, our slang is so unconscious that we do not even realise we are speaking it. Since I live Rural and remote I probably hear and use it more than Sally does. I am constantly having to explain what certain words and sayings mean
Every country or region I have lived, there has been local slang. And, it often changes among the young on an ongoing basis, or just when the older folk catch on. When I was being raised in Belfast, I spoke the North dialect of Irish, and we made distinction between English and American pretty much as equivalent to different spoken and written dialects. Then, as for the slang... lots different local variation in Irish as well as English and American... then there is Cockney. In the US, there are regional and local variations of slang, not always recognizable among those from different areas. Sometimes makes for interesting conversation. I had a friend from home visiting me in the US, one evening at an early evening gathering, he suggested we go find some good craic... interpreted by the Americans in the group as crack...led to a bit of misunderstanding.
When I was in college I taught a machine shop course at the local community college. It was four hours long and by the end I was speaking in the same rural drawl as my students. One semester I had a class at the University immediately following my lab course. It was a graduate course in educational psych and it lasted two hours. I'd still be speaking with the drawl at the beginning of the class but as it progressed my diction reverted to major university educated individual. My classmates thought I was odd.
I have noticed over my time living in the US (or anywhere for that matter) I unconsciously adapt my use of language and even my accent to adjust to that of those I communicate on a frequent basis. A long time ago I realized the changes were in response to unconscious (some time conscious) I picked up on how well I was being understood. So while my vocabulary was modified (extended) and my accent muted at times relative to the communication environment I was embedded in, I realized the difference was a learned behavior based on how well I was being understood or not. When going home I also noted there was a lag in the shift of my communication language/accent and when returning to where I was living my friends almost always would comment on being able to detect I had traveled home. Funny too, after being home a bit, and traveling to other parts of Ireland, people will often correctly guess the area I was raised, with some even specifically able to identify the city neighborhood. In any place or organization there is always a subculture of differences reflected in communication style, language and operational vocabulary. For instance, visit any medical organization and note how communication style and vocabulary changes (think in terms of the use of medical jargon and specialized acronym shorthand’s). In my field, there are loads of both specialized jargon and acronyms that an outsider would find difficulty in following a conversation. Then, consider, if you accept Noam Chomsky’s early theories of linguistics, he would suggest language also impacts cognition as it becomes embedded in the way we encode information about the world internally... an area of study I am currently engaged in studying.