Alternatively - And this is my headcanon...
What if the mortal races could be 'Converted' to Half-elves at any time (wwoooo alien abduction and assimilation!) through a series of cybernetic modifications that the race usually install at birth (from pods, because, of course) to make them 'better beings'.
The resistance to magical charm; parts of the brain are replaced, half the brain is unaffected by magic like the full bodies of fully converted Farang. These replacements make the mind more efficient - granting Skill Versatility and the ability to suddenly be able to piece together information to learn language - one of which, Elvish, being 'auto installed' on the hardware from the start actually requires a laryngeal replacement to modulate the sounds you make because for 'common' speaking races it is incomprehensible sound thats why the courts only vaguely understand the written language.
Honestly, I have been stealing and building on this for so long, I have so many random ideas.
But thats fanfiction - this is a thread of FACTS... So this WHOLE POST was a waste of time.