When strings handled by the jsbackendauthority contain non-ASCII, the
code will fail.  For example, on a system having a user with a
non-ASCII name, the following message will appear when a USB stick is
plugged in.

mar 04 21:47:31 mimmi polkitd[17163]: Error evaluating authorization rules

The user will not be allowed to do the mount.

The problem is that strings were variously encoded back to C strings
with JS_EncodeString and JS_EncodeStringToUTF8.  According to the
documentation
(https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_reference/JS_EncodeString#Description)
the former will simply drop the high byte from each character.  If
that happens to a username, it will no longer be found as a valid user
name on the system.  Explicitly encoding to UTF-8 will at least work
in UTF-8 locales, which is the increasingly dominant encoding.