Minecraft had been working fine on the new launcher for roughly three months and all of a sudden had an error saying a lang file was missing. As a test I downloaded the old launcher again, and the game launches successfully on it. It seems to be a problem with the new launcher.
Full reinstall did not help.
I have seen that many reports like these are resolved and blamed on a program called ByteFence. I do not have this program. This is not ByteFence

Error message:
Name: cs_cz.lang
URL: http://resources.download.minecraft.net/54/549a0701b4db03659fb974f7676e95e6bf46102b
Error details: Request error -101: ERR_CONNECTION_RESET
Filename on disk: f551-ddee-fea1-4bb6
Path: C:\Users\John\AppData\Local\Temp\f551-ddee-fea1-4bb6
Exists: file