Does anyone know what you're talking about if you say diaper? I've had two kids here and just naturally used the word Nappy - I just assumed no one would understand the word "diaper"
I would have thought that most would understand diaper. Decades of exposure to American TV and movies has ingrained a lot of terminology in British minds, even if some people object to what they see as American "corruption" of the language. I certainly knew the meaning of the word diaper many, many years before I ever visited the United States.
There seems to be a problem where the same word is used to mean something different, though. For example, you can bet that most Brits will understand "sidewalk" perfectly and translate it into the British equivalent "pavement." By far the majority of those people, however, will
not know the American usage of "pavement."
I still have to get used to the word garage..or as it's properly pronounced here..gar-ridge.
There is an alternate British pronuniation which seems to be on the decline these days. It's much closer to the normal American pronunciation but with the stress on the first syllable instead of the second. Take the emphasized "GA" part from the common British version and add the "rage" as in American but without the stress, and you'll have it.
I also have to get used to cookies being called biscuits (which is also what crunchy pet food is called)
There are certain topics with quite widely differing terminology, anything to do with food being one of them. Cars and trains are other fields where you'll find considerable differences (compare trunk/boot, hood/bonnet, muffler/silencer, railroad ties/sleepers, conductor/guard, etc.).