lang:en_US capitalisation in options.txt prevents resources from being loaded

The bug

The capitalisation of the line lang:en_US in the file options.txt (as found in profiles created in MC 1.8) prevents any resource from being loaded, therefore even the start menu displays the pink-black "missing texture" texture only.

Symptons are described in bug MC-122148, but the cause was misidentified (it has nothing to do with third-party resourcepacks being used) and the bug was not reopened, when @unknown pointed out the root cause (kudos to him/her). Hence this new report.

How to reproduce

1. Create a new profile for MC 1.8

2. Start this profile and change an option, such that options.txt is being written to disk, then close game

3. Verify, that the options.txt file contains a line lang:en_US or similar with uppercase country code

4. Change profile to use an affected snapshot version, e.g. 17w48a

5. Start the game

The fix

The parser of the options.txt file needs to be able to deal with the capitalisation as found in MC 1.8 profiles, especially the language codes which are e.g. en_US in the older and en_us in the new version of Minecraft profiles.