I just finished one that had “Mussorgsky” and “Instrumentation: Ravel.”
I thought about abbreviating it to “instr.” but in the end I didn’t. I’ll fix that later, if that’s the decision.
I feel like there should be a comma or something between the composer and the instr./arr./orch. credit, but that’s just me.
There’s no decision yet, to be clear, that’s just a proposal to see what others think 
I did consider a comma but it didn’t seem to add much - if people really want one it’s certainly an option though.
This all seems reasonable to me, especially for releases in English.
I agree with VBZPPINQyJ. At lease for English language releases, prescribed abbreviations, and not too much detail, seem like the best way to go.
I went ahead and abbreviated “instrumentation” on the release I mentioned. I know it hasn’t been decided yet, but it seems to be the way folks are leaning.
I think the ambiguity is my main reason for why I wouldn’t have them in track credits. Sometimes they’re credited and sometimes they’re not on releases. Sometimes it’s credited but in a very minor way. Nearly all languages would differ if we use a word like arr.. The classical artist credits currently works as a list system with separating punctuation. It is generally understandable for any script or language. Adding a word tied to a particular language is not a good idea, as it kinda goes against the universality of the classical formatting.
Given that there has not been any mention of it in the guidelines, most releases wouldn’t have arrangers right now. And in the past I’ve removed them on sight because that’s what the guideline made me believe was the correct way of dealing with it, as a relationship but not part of the artist credit.
For J.S. Bach arr. Webern, the most common form would be something like J.S.バッハ（作曲）／ウェーベルン（編曲）. Which directly translated to English would be J.S. Bach (composer) / Webern (arranger). The more compact form would be J.S.バッハ（ウェーベルン編）, J.S. Bach (Webern arrangement). You wouldn’t insert a word in between the composers and arranger, as the word for arrangement is suffixed usually. It should be noted that I am not super proficient in Japanese, but this is usually how I see it listed.
Should works with a special-purpose composer be treated differently? E.g., the tracklist below looks like Joseph Canteloube should be the track artist, but Song-cycle “Chants d'Auvergne” - MusicBrainz says he’s the arranger and the composer is [traditional].
Edit: added the second image because it makes it clear that he’s the arranger.
I think in this case I’d just follow the release and stick to Canteloube only, while if it said “folksong arr. Canteloube” in big on top I’d also credit [trad]? But I think there’s always going to be edge cases and I wouldn’t complain if someone did it with both artists here as well.
