The rule of thumb is, if the word or short phrase has been so commonly used in English writing that it's immediately recognizable to an English reader, meaning the word or phrase has been assimilated into current English usage, then don't italicize it (i.e. it is anglicized and should be set in roman). If the phrase is an abbreviation of a Latin phrase, also don't italicize it. Finally, don't italicize foreign proper names, foreign names of people, institutions, places, etc., and also usually not for foreign quotation.
Otherwise, italicize the word or phrase to show that it's foreign. When writing italicized foreign words or phrases, make sure to put in the appropriate accents and diacritical marks (this means that for German nouns, the initial letter is capitalized.
But which words or short phrases are commonly used in English enough not to be italicized? It could be a tough call. To help you, here's a compiled listing of frequently used foreign phrases and whether to italicize them:
|Common Foreign (e.g. Latin) Phrases and Whether to Italicize Them|
|Do Italicize||Don't Italicize|
|ex ante |
cabinet (French type)
coup de foudre
If a word or phrase isn't in the above list, check the New Oxford dictionary for writers and editors for whether or not to italicize it, or check to see if the word or phrase is in the Merriam-Webster Online (if it is, don't italicize it).
The above listing was compiled from the following three sources, which are my "go to" sources for all my formal writing style guide needs because they are free and professional: