++The JSON file /assets/indexes/1.12.json has a typo in the English language file name, which I discovered upon loading up a working Forge modpack (All The Mods 3) after a client update when the game crashed on load because of 2 mods that use the launcher assets for their translations which failed because of the typo.

The offending entry (Lines 211 - 214) is as follows:
"minecraft/lang/en_ud.lang":

{     "hash": "66d650980b69dde6eb492b61844b6c53328a5442",     "size": 204238 }

,

 

Clearly, this was supposed to be the following:
"minecraft/lang/en_us.lang":

{     "hash": "66d650980b69dde6eb492b61844b6c53328a5442",     "size": 204238 }

,

As confirmed in this issue report's comment by mcdodge34