One way to do this may be a Timeline-191 esque scenario where the U.S. allies with a non-English European power, likely either Germany or Russia, and takes loanwords from the language. Over time due to linguistic drift, American English loses intelligibility with its British counterpart, eventually becoming it's own language; it would still be a West Germanic language, perhaps even creating a new branch of the Indo-European language family, the Brittanic languages. If the progression of technology is the same as in OTL, that would also affect the development of American English dialects through exposure to non-Anglophone foreign linguistic influence in mass media.