Research the correct spelling and interpretation of the word or phrase in a foreign language dictionary before including it in your writing. Don't guess at the meaning, as foreign phrases are sometimes used incorrectly by nonnative speakers. Formal usage and conversational usage are different, so use a conversational dictionary as well or check with a native speaker, if possible.
Include foreign words or phrases in your writing, but don't explain the usage. Let the context of the foreign word or phrase do the explaining for you. This strategy works well for fiction, but only if you use it sparingly. Too many passages using too many foreign phrases and words becomes an obstacle course for the reader. Trust your readers to research a translation if they find it necessary to understand the plot.
Write the word or phrase in a natural manner, and set up an appositive. That is, rename the foreign words, in English, in a parenthetical phrase. For example, a passage might look similar to this: "She had a certain "je ne sais quos," an indefinable charm, that illuminated the room." In this way, you gracefully work in an interpretation.
Set foreign words in italics if they are commonly not found in an English dictionary. This technique also helps draw attention to their usage.