What's the locale for system files (e.g. /etc/fstab)?

From: Phil Shafer <phil_at_freebsd.org>
Date: Wed, 23 Mar 2022 20:31:08 UTC
On 23 Mar 2022, at 11:51, Piotr Pawel Stefaniak wrote:
> mount: make libxo support more locale-aware
>    "special", "node", and "mounter" are not guaranteed to be encoded 
> with
>    UTF-8. Use the appropriate modifier.
> -       xo_emit("{:special}{L: on }{:node}{L: (}{:fstype}", 
> sfp->f_mntfromname,
> +       xo_emit("{:special/%hs}{L: on }{:node/%hs}{L: (}{:fstype}", 
> sfp->f_mntfromname,
             sfp->f_mntonname, sfp->f_fstypename);

This recent "mount" patch highlights a libxo-related problem for which I 
don't have a solution:

There are several files for which the encoding is not known.  Since 
locale is user specific, we don't know how to interpret the contents of 
/etc/fstab.  It's assumably been encoded with the format of the user who 
wrote it, but that information is lost.

Put more generally, there's not a system-wide place which declares the 
encoding for system files, which leads to this problem where we 
interpret files from one user's locale using another user's locale.

One solution would a symlink in /etc that "points to" the name of the 
current system-wide locale name.

% ls -Fl /etc/locale
lrwxr-xr-x  1 root  wheel  7 Mar 23 15:42 /etc/locale@ -> C.UTF-8

(Or "/etc/system.locale" ?)

If the symlink doesn't exist, would "C.UTF-8" be a suitable default 
moving forwards?  It certainly would not be backwards compatible, since 
an existing fstab could have non-UTF-8 strings in it, encoded with the 
locale of the user who touched the file.  But there's really no 
backwards compatible solution, given that there's no guarantee that (for 
any specific FreeBSD system) all system files were written with the same 
locale.  Fun, eh? ;^)

Opinions, thoughts, please?