Or are the lyrics in some kind of language?
Maybe a fictional language constructed by Yoko Kanno (AKA Gabriela Robin), possibly also used in her other works that use similar lyrics? Or maybe puns/wordplays, as described as special case for Japanese music in Word Salad Lyrics, "based on alternate translations of the kanji used" or "similar-sounding words"?
What I found
I found many claims that it’s not in any kind of language, but none of them are from authoritative sources, and none of them cover the possibilities that it could be a fictional language or Japanese wordplays.
Xah Lee claims on his page about Moon:
As with many of Yoko Kanno's songs, the lyrics is meaningless vocalizations, not of any language.
From Gabriela Robin’s article in a wiki about Yoko Kanno:
Her very characteristic is to often write lyrics in a specific language she seems to be the only one capable of translate.
From Yoko Kanno’s article in TV Tropes (but not in particular about Moon):
Word Salad Lyrics: When she writes lyrics, this can happen. And sometimes what sounds like word salad is actually a nonsense language.
In the thread What language is this song? in the LoadingReadyRun forum, various languages are suggested, and some claim that it’s just gibberish. A source (supposedly a fan page, which I can’t locate) is quoted:
Her non-sensical lyrics have been known to drive the unknowing fan searching for translations from different linguists, but to no avail.