In my resource pack that I created, I had a custom language file. It got rid of ellipsis I didn’t think were necessary, replaced all ampersands with “and”, and a bunch of other stuff.
So when I went into the language selection menu, and chose Pirate Speak, and then switched back to English US (already selected when I entered), my custom language file was not selected.
I unselected my resource pack, then headed down to 1.6.4, where I tried to put it on to see if it would work. It didn’t. Then I deleted the “lang:” field of text in the options.txt, and tested it out on 13w42a, and it didn’t work. I decided to delete the options.txt, and it didn’t work.
I took some screenshots showing that I put the language files together right and forced a crash.
Is this still a concern in the current Minecraft version 1.7.2 / Launcher version 1.3.3 ? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.