Italics are awfully overloaded. I never italicize foreign terms, because that makes them even harder to read, and for mentioning a word as a word, I prefer double quotes, which provide clearer delimiters.
If you insist on using italics for emphasis and for foreign terms, you can use the rule that nested italics cancel each other out: (I dare not use a GameFOX quote box here, since GameFOX quote boxes are implemented with <em>, which normally yields italics)
"Now consider the tlatoani. The tlatoani…"
Here, the first instance of "tlatoani" is doubly italicized. If you use a foreign term more than once, chances are you won't want to emphasize it every time, so the reader will notice the difference, even if they can't figure out what it means.
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"If I pick nine, you'd better not be racist against me!"
"No promises, niney."
If it's a single non-English word in an English sentence, for instance, I doubt that the single non-English word is the one that will need to be emphasised any further.
If you're emphasising a non-English word as part of a non-English sentence in an English work, then you could probably just un-italicise the non-English word in question.
That was much more pedantic than I would have liked it to be.
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x is an irrational number if and only if the set { nx mod 1 | n is a natural number } is dense in [0,1].