Ah, thank you for the explanation
I was in fact originally hoping to get that - I didn't quite grasp that it might not be possible, because I use plenty of non-ASCII folder names so it's not something I'd thought about. I didn't realise "within its own library" meant folder names, I assumed it was referring to something in the Calibre software. But I worked it out as I went along, and correct romanizations are a definite step up.
On a conceptual level I feel it's a shame transcription is forced in this way, because it does lead to some problems. There's this bit of confusion, it'll still fall down on names with variant pronunciations, and it's going to be a bit of a pain combining Chinese and Japanese ebooks. It's also slightly awkward if you want/have to deal with folders rather than only touching them via Calibre. Ah well.
I'd probably have aimed to include a "transcription" field that lets you auto-generate a transcription or create your own. Those of us who are quite happy having non-ASCII folder names could keep them. But that might be a lot harder than I realise, and I appreciate the difficulties with trying to build robust software with maximum compatability. Just a bit of musing really