[Koha] Vietnamese language choice in OPAC appears in Chinese characters

We have used Koha in Vietnam for several years, and when the vi-VN locale is installed, it appears in the OPAC as Chinese characters. Not the actual interface, mind you, just in the language selector: <a href="/cgi-bin/koha/opac-changelanguage.pl?language=vi-VN" tabindex="-1" role="menuitem" class="listmenulink">㗂越</a> That seems to be a bug to me, but maybe there is an explanation? Is there a way to fix it? It should be "Tiếng Việt" not "㗂越". Daniel