Say I have a word from another language that I am very fond of and I want to just plop it in a sentence without trying to translate it. Should I use italics? Example:
I've never cared for winter sports because I'm friolenta -- I get cold easily.
Obey your manual of style, either the one you chose or the one thrust upon you. I use The Chicago Manual of Style, which says that unfamiliar foreign words should be set in italic. The exception is proper names so it's Moscow in English and Moskva in Russian, but both are in roman type.
In deciding how familiar is familiar, CMS says that "there are no guides but sensitivity and common sense."