If you ask any American what language they speak in their country, in most cases the reply will be that it's English. However, American English sometimes has a completely different vocabulary to British English.
Many Americanisms have entered the language on this side of the Atlantic, but they are a long way from being established. It's quite strange how some Americanisms have been accepted but others have completely failed to make any impression on British English.
On our side of the pond we say: apartment, ball park, buddy, cab and cookie. However, we don't say: pacifier, realtor, popsicle, duplex and gurney. (A British English spell checker objects to these words.)
The way words are used is also different sometimes. For example if you ask an American how they are, you will get the answer, 'I'm good!' In English English we say, 'I'm well.'
Among the young in Britain, there are those who regard American English as 'cool', but their use of this alien (to us Brits) tongue causes annoyance amongst those who don't.
For more information on this and examples of how British English varies from region to region, you might find these articles useful.