@@@ Helge Fauskanger suggested that Eldarinwa is an extended form of this word as an adjective (QQ/Eldarin). More exactly, I think it is the application of the possessive/adjectival suffix -va to the word Eldarin in its noun-sense = “Elvish language”.
eldarinnoun/adjective. Elvish, Elvish (language), Elvish languages, of the Eldar
-ya (5) adjectival ending, as in the word Quenya "Elvish" itself; when added to a verbal stem it may derive a kind of short active participle, as in melumatya "honey-eating" (mat- "eat"), saucarya "evil-doing" (car- "do"). (PE17:68)