RDBMS's have gone through the same journey. First it was hardcoded, now for many of them we can specify the codepage or UTF encoding and collation of a DB, some even offer it per table or column, and so on.

Filesystems should do the same. Either pick one way and stick to it (expose it as UTF8, with a sensible normalization and collation) or offer several options that can be specified when creating a volume and do on the fly conversions for clients that need it.