Italicize:
* Words not commonly used in English: If a word is not widely understood or used in English, italicize it on its first appearance. For example: "The *schadenfreude* of seeing the rival team lose was palpable."
* Words that are being used for emphasis: If you're emphasizing a word's foreign origin, italicize it. For example, "She used the word *faux pas* to describe his behavior."
* Titles of foreign works: Titles of books, movies, songs, etc., in a foreign language should be italicized.
* Latin phrases: Many Latin phrases are italicized, especially if they're not commonly used. For example, "He approached the task with *de facto* authority."
Do NOT italicize:
* Words that are commonly used in English: Words like "bon appétit," "ad hoc," "de facto," "a priori," "zeitgeist," "schadenfreude," "chutzpah," "laissez-faire," etc., are considered English words and are not italicized.
* Proper nouns: Foreign names of people, places, or things are not italicized. For example, "Marie Curie," "Paris," "The Louvre," "The Taj Mahal."
* Foreign words used as adjectives: Foreign words used as adjectives are not italicized. For example, "The *haute couture* gown was stunning."
* Foreign words that have been Anglicized: If a foreign word has become a regular part of the English language and is spelled like an English word, it's not italicized. For example, "The *pizza* was delicious."
General Rule: If you're unsure, it's always best to err on the side of caution and italicize the word. However, if you're writing for a specific publication, it's best to follow their style guide.