Wow.
There are a lot of things. So far I have only been on extended visits, but as my husband was 'fresh off the boat', as it were, when when I met him here in Atlanta 7 years ago, there was a LOT to get used to, language and culture-wise.
I spent a lot of time rolling my eyes at my future in-laws on my first visit. As a person with extreme allergies [and the hubs' are even worse than mine!], I was shocked at all the mold, dust and general dirtiness of everyone's house. Even 'clean' folks never washed their duvets or used mattress or pillow covers. I'm not a clean freak, but with my allergies I have to, ya know. I suppose my worldview is somewhat skewed on this matter, but Everyone I visited [with the exception of my father in law], had houses so dusty and moldy and dirty that I couldn't bear to be there for too long.
And don't get me started on the dishwashing thing. You rinse the dishes not to make them 'cleaner' but to wash off the toxic SOAP you just put them in. Don't these people realize they're all dying of cancer because they've been eating their food off of chemical covered plates all these years? I don't mind the dirty teapot

At least it doesn't have dishsoap residue in it. [And I only get huffy about this because I've spent years eliminating chemicals from my life and then my MIL got cancer and I got on her about the rinsing thing].
Speech and Language: My husband is from East Yorkshire, so I'll add a few I've had a hard time with...
Dinner is what they call Lunch [MIL: 'At school we had dinner!'], so
Tea is what you eat in the evening. Although they do say supper if it's late.
But I still do a face when my MIL pesters our kids to 'Eat your Tea!'
As for the Trolley, pram, and other wheeled things, I've managed to get away with using the word Buggy for all of them, which we do here in GA and it's understood.
Other things I've never heard:
Ta for Thanks
Cuppa for an afternoon cup of tea [or even coffee]
Pissed is drunk, of course.
Sick is puking.
Poorly is the word they use for being unwell or what we'd call sick.
So many others. Just the unintelligible way my hubs says certain words, still, after 7 years in the US. He's better, but he's still the master of the the non-committal grunt [which my girls have learned],and I still have to regularly ask him to 'spell it' if I don't understand what he's said.