If you use an unfamiliar foreign word or phrase in an English sentence, you should italicize it.
You do not need to italicize foreign words that have become part of the English language—“laissez-faire,” “fait accompli,” “modus operandi,” and “per diem,” for example.
One test for whether a word is foreign or a part of English is to look it up in the dictionary. If the word or phrase is an entry in a English dictionary, you don’t need italics.
Exercises:
Italics 1
Italics 2