++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
Thank you for your report!
However, this issue has been closed as Invalid.
en_ud is for upside down English, there was no typo
P.S. this is a Minecraft Forge issue which has already been fixed