- The foreign word (other than proper names) is italicized in an English language work if it has not become part of the English language, either for the first use only or all the time – it depends on your publisher's guidelines.
- A foreign word that has been absorbed into English is not italicized. You'd be surprised how many have made the transition, so I always check the Oxford or Webster dictionaries to see if the word is listed.
I visit France often and regularly face this issue when writing about my travels there. The French word for castle is château, but it is now considered an English word too, has lost the circumflex, and would not be italicized. Merci is not in the Oxford Canadian Dictionary. Metro is anglicized. Aurevoir is not.