I think I may have a name for the Engadine in Engadinese itself: Änzip. (That’s the “Romansh” spelling; “Croatian” would be Æncip and the native spelling Αίντθηπ < Αίνος “Inn” + κήπος “garden”. Pronounced /’Ents)ip/ or thereabouts.) That’s assuming that *ki- > tsi- (as in rm-rg “citad” < CIVITATEM, for example).
I’m still not sure how to derive language names, though, so I don’t have a good word for “Engadinese” yet. Two possibilities I’ve considered are -ιστί (> -ští?) and -ικός/-ικώς/-ική (> -ič? -ičó? -ičús? -ić-?). Though something parallel to “Romansh” is still appealing, i.e. derived from “Hellenic” or “Attic” or “Koine” or “Ionic” or “Byzantine” or whatever.
Hm. I wonder what would make a good language-name morpheme à la -ish or -ese in English.