As far as I understand language tags get validated by IANA registration of being proper standard compliant tags. If I have a given tag like "mn-mong" how do I go about looking up the meaning of the tag in the registry?
You can look these up at IANA's own site:
In "Tags for the Identification of Languages" [BCP47] [RFC3066] there is a provision for listing unique "tags" or names for languages and variants of languages. This document summaries the list of assigned language tags.