Wiktionary defines
isolect as:
(linguistics) A language or dialect; coined as a neutral term between ‘language’ and ‘dialect.’
Apart from Wiktionary I didn't see it in any general dictionary. I also didn't find it used anywhere but in scholarly publications. This, of course, doesn't necessarily prevent it being accepted into Chihuahua as a rare word. Usually when a word is only used in specialist writings, its meaning is difficult to comprehend by the lay reader. But in this case, the meaning and usage are quite easy to understand.
I'll accept isolect as a rare word.