The phone below has a Choose language menu in which you can choose your language from a multitude of languages and variants, including 日本語（アメリカ合衆国）:
QUESTION: Is there really an American Japanese language variant?
If yes, how different is it from Tokyo Japanese?
The title of the dialog is Change language, and units/date format/first day of week/etc are covered by a different dialog.
Another choice is Italiano (Sudafrica). Even though it might sound surprising, there actually is a variant of Italian in South Africa.
A Google search shows that the ja_US locale seems to be used by quite a few people. Example taken from a technical mailing list (I don't imply that this person is right, I just say that this point of view exists):
you can configure Wine to display Japanese by using the LC_ALL enviroment variable. In the United States, set it to ja_US, and in Japan, set it to ja_JP. The reason for the localization difference is because programs compensate for the different dialects of a language across different nations (think en_US and en_GB, one for the US and one for England.)
No, there is no such thing as "American nihongo". As the comments have suggested, we can assume this has something to do with "locale" settings, such as date formats (although it's hard to see how writing dates in American would actually help Japanese speakers), and perhaps things like units (where Japanese speakers in America will perforce be using "American customary" units like 'inch' and 'gallon'). It helps if you understand that 99% of the people involved in so-called localisation have no clue how language works, and so you can expect that this stuff will not make sense. (My computer is set to use something called "Danish English", because that's the most usable option I have found; I am English and live in Japan.)