Crash when language file of currently selected language is missing

When the language file of the currently selected language is deleted on the harddisk, the game crashes on startup.

What I expected to happen was...:
To fall back to en_US

What actually happened was...:
It crashed.

Steps to Reproduce:
1. Make a copy of a language file, e.g. ./minecraft/assets/lang/aa_aa.lang
2. Select that new language ingame
3. Stop Minecraft
4. Delete the language file
5. Go to sleep (not sure if step is necessary )
6. Restart Minecraft the other day and wonder about the crash