The lyrics of Moon seem to be in no (known) natural language:
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.
These claims don’t seem to be implausible, however, these are not authoritative sources and they don’t list any proofs or other sources.
I can see two other possibilites:
TV Trope’s article about Word Salad Lyrics states:
A special case is Japanese music. A lot of apparently incomprehensible Japanese lyrics are actually puns or other wordplays based on alternate translations of the kanji used, similar-sounding words, or (most often) both.
This could apply here, too, as Yoko Kanno is Japanese.
It could be the case that the lyrics of Moon are in a fictional language, i.e., with a meaning, even if it’s not documented anywhere.
As Yoko Kanno (AKA Gabriela Robin) seems to use similar lyrics in several of her other works, it could even be the case that it’s the same fictional language in all these works.
Are the lyrics of Moon meaningful (i.e., in some kind of fictional language, or wordplays, or similar), or are they nonsensical?