The tag implication #51967 mongolian_text -> cyrillic_text has been rejected.

Reason: Cyrillic letters are used

EDIT: The tag implication mongolian_text -> cyrillic_text (forum #377742) has been rejected by @gattonero2001.

As per the wiki:

cyrillic_text should only be used when the Cyrillic script is being used nondescriptly (i.e., not of a particular language)

Also, for future reference, you can make a BUR to do multiple related aliases and implications together at once.

What if the characters are transliterated but the words are still obviously mongolian/russian/etc.?

On top of Watsit's point, it's not guaranteed to be in Cyrillic.

You've also got the Mongolian script which lasted thousands of years, as well as the short-lived Mongolian Latin alphabet - I wouldn't be surprised to see either of these two things, considering the long history of the Mongolian script and the likelihood the Latin alphabet might be used to be more understandable for a western audience.

This argument especially goes for other languages you suggested like Serbian, which is well known for its speakers being diagraphic and commonly using both the Cyrillic and Latin script in everyday use.

The tag implication mongolian_text -> cyrillic_text (forum #377742) has been rejected by @gattonero2001.

llorando said:
The tag implication #51967 mongolian_text -> cyrillic_text has been rejected.

Reason: Cyrillic letters are used

EDIT: The tag implication mongolian_text -> cyrillic_text (forum #377742) has been rejected by @gattonero2001.

This would just cause a lot of tag bloat and isn't guaranteed to be accurate.