I hadn't run minecraft in a while, and I launched it, intending to use the new version. I clicked the "download 1.11.2" and expected it download and play.
It failed to download, so I shut it down and restarted the launcher. When it didn't work again I tried playing an older version (1.9.4) and it worked fine, so I tried again and it still failed to load that version.

As for the steps to reproduce
I just launched minecraft and the version wouldn't work.

it gave me the following error message:
Error
unable to save download. de_de.lang
Unable to save download. de_at.lang
Name: de_at.lang
URL: http://resources.download.minecraft.net/31/319c12b129219f6ae14e5f565b103760ca45f4fb
Error details: Attempting to move successful download to C:/Users\theac\AppData\Roaming\.minecraft\assets\objects\31/319c12b129219f6ae14e5f565b103760ca45f4fb
The process cannot access the file because it is being used by another process
Filename on disk: d61e-e0b0-87af-2462
Path: C:\Users\theac\AppData\Local\Temp\d61e-e0b0-87af-2462
Exists: file