[MCL-5837] Game versions 1.5.2 and lower crash when starting in a newer version's folder due to invalid language in options.txt

Workaround

Downgrading is not, was never, and will never be officially supported. You are advised to run older versions in separate directories. Otherwise, issues like this will occur.

To change directories, please read this guide.

If you need to use the same directory (not recommended, but provided for completeness):

Stop Minecraft
Navigate to .minecraft/
Open options.txt with an editor
Search for the line lang:xx_yy (xx_yy = LanguageCode_CountryCode, e.g. en_us for English (US) )
Change the lowercase CountryCode to uppercase, e.g. change en_us to en_US
Save the file

When launching Minecraft 1.5.2 or below in a folder that was previously used to start a recent version (e.g. 1.12), the game will crash on start:

java.lang.NullPointerException
 at java.io.Reader.<init>(Unknown Source)
 at java.io.InputStreamReader.<init>(Unknown Source)
 at bp.a(SourceFile:64)
 at bp.a(SourceFile:100)
 at net.minecraft.client.Minecraft.a(SourceFile:252)
 at avv.a(SourceFile:56)
 at net.minecraft.client.Minecraft.run(SourceFile:507)
 at java.lang.Thread.run(Unknown Source)

Steps to Reproduce:

Create a new profile using the latest version.
Launch it.
Close the game, and then change the version to 1.5.2 (or earlier).
Launch it.
Observe that the game crashes on start.
3. Watch it not even start up and crash.