yes,i followed the wiki,but the language code <mn> is Cyrillic Mongolian ,its defined in here http://opengrok.libreoffice.org/xref/core/i18nlangtag/source/isolang/isolang.cxx#388 and http://opengrok.libreoffice.org/xref/core/include/i18nlangtag/lang.h#300 <http://opengrok.libreoffice.org/xref/core/include/i18nlangtag/lang.h#300>
Well https://en.wikipedia.org/wiki/Mongolian_language <https://en.wikipedia.org/wiki/Mongolian_language> defines “mn” as the official code for mongolia, so I assume that is why we use it. There are also quite some translation work done on that code.
Is is possible that vcl don't recognize traditional Mongolian?
If “mn” is not traditional Mongolian (which it seems to be) you need to find the proper iso code. A new code need to added several places in the code being as described in the wiki page you first referenced. Please have a discussion on the l10n mailing list, about which codes should be used, as Developers we look at the ISO-639-1 definitions. rgds jan I.