How should be the Japanese common noun expressed in English?

In English, common nouns are never capitalized unless they begin a sentence. This is also true of common noun words borrowed from other languages. If the borrowed word is unusual in English, enclose the foreign word in quotation marks and, on first usage, be sure to define the word in the text.

English, especially American English, borrows words left and right, and many -- like sushi -- have been fully incorporated into the language. These words should not be enclosed with quotation marks.  Check a good dictionary or use Oxford Online to determine whether the word is recognized in English.