Mumble 1.2.3 doesn't currently show X509 certificates that include UTF-8 strings as part of the subject and/or issuer names.
I recently updated MumbleKit to correctly generate UTF-8 certificates: https://github.com/mkrautz/mumblekit/blob/master/src/MKCertificate.m,
and when I import these certificates into Mumble, the UTF-8 parts show up as unknown characters.
Windows and Mac OS X certificate viewers correctly show the strings.
Attached is an example of such a certificate.