Latin, as a general rule, has two different types of words. The first type inflect fully, and for adjectives that means a full set of case, number, and gender forms. The second type don't inflect at all and use the same form for every case (usually these are only foreign names). And as far as I know, every adjective in the language falls into the first category: there's no adjective which doesn't have a full case-number-gender inflection.
So while the noun "Aztec person" might be borrowed as Astecat (undeclined) or Asteca (first declension), the adjective would have to decline in some way. Either Astecus -a -um or Astecicus -a -um would make sense, though I'd go with the first (back-formed from the noun Asteca) to avoid the pattern -ecic- in the middle.
EDIT: Brianpck has pointed out the adjective nēquam "worthless" is, in fact, indeclinable. I stand corrected! But I believe my point is still valid for foreign/borrowed adjectives.