Currently the new <langconvert> tag (T263082) uses MediaWiki internal language codes rather than BCP 47 codes. In most but not all cases these codes are the same. <langconvert> should be fixed so that it accepts BCP 47 codes such as sr-Cyrl and rejects internal codes such as sr-ec.
I'm going to work on this and submit a patch soon. It shouldn't be too hard but I want to be careful to cover all of the edge cases.