If you are quoting someone's exact words, and you want to quote them in the original language for the sake of precision and accuracy, just put them in quotation marks as usual and follow them with an English translation in parentheses: "When Marie Antoinette heard that the peasants were rioting because they had no bread, she supposedly said 'Qu'ils mangent de la brioche' (let 'em eat cake)." If you want to use a foreign-language expression that is well-known to English speakers, put it in italics. I'll show italics here with underbars. "There did not seem to be anything distinctive about him, yet he had a certain _je nais se quoi_ that appealed to almost everyone." If you want to use a foreign-language expression, because it expresses an idea better than any word or phrase in English, but you do not think your English-speaking readers will know it, put it in italics and explain it. "She had the quality that the Spanish call _duende._"