Several Cyrillic alphabets are available for numbering headers in Tools→"Heading numbering"→Numbering→"Number:", but Cyrillic Mongolian is not there. The Mongolian Cyrillic alphabet is there: https://en.wikipedia.org/wiki/Mongolian_Cyrillic_alphabet#Description . It's the official alphabet of (outer) Mongolia. I guess it would not be difficult to add it. An alternative would be to allow the user to define an alphabet he/she wants to use for such a purpose. (See also bug#115189 for the Mongol-Uyghur alphabet)