Runner's Companion lists "Upvehu" as a language commonly spoken by Pixies.

Both my Shadowrun knowledge and Google fail me. Anyone know anything about Upvehu ?

Made up language. Meant to expand on it.

Cool.

Is it an isolate, or is it related to any other languages ?

Do some of them remember it themselves (somehow), or was it preserved by dragons or elves ?

Effectively a language isolate at this point, maybe some very distant connections to proto-Indo European, a word or two of similarity with Sperethiel is not impossible. Different populations will have their own dialects, so Portland pixies and Breton pixies would be distinct, at least by idiom and accent.