There is no U.S. Cantonese and Mandarin in the sense of Swiss German, American English or Pidgin English. Even "Cantonese and/or Mandarin Chinese used in the U.S." is not an established term. I can only say that when translating from a foreign language into Chinese -- (actually in writing, there is not much difference between Cantonese and Mandarin, but in spoken Chinese, a great deal, between the southern Chinese dialect of Cantonese and Mandarin which is used in a much larger area by probably a majority of the Han Chinese) -- for use in the United States, it might be more important to find out who the audience, or readership will be. For instance, (1) young and new students /academics or new immigrants from China north of Canton -- simplified Chinese and mandarin; (2) old immigrants from southern China, middle aged or older workers in academic fields -- traditional Chinese and mandarin. etc.