With some tinkering, I managed to make minecraft 1.14.4 run on the standard raspberrian image! No need to install gentoo then if you don't have a spare sd card, have raspberrian already and want to keep it. Ok, I will now try to explain what I did, hopefully remembering everything. (Only follow the following if you are using standard raspberrian, use original post if you are using gentoo)
I started off with increasing GPU memory from defaul 64 to 256MB in settings, and rebooted.
I then think I ran step 1 and 2 in this tutorial.
I then did this either after step 2 or in step 4: As said in the other post, I then had to download the old launcher from from http://move.rupy.se/file/launcher.jar
and I then ran java -jar launcher.jar instead of java -jar Minecraft.jar
I installed Optifine like in step 3
In step 4, remember to use launcher.jar instead of Minecraft.jar
In step 5, use nano or emacs instead of mousepad if you don't have it (It wasn't included by default in my raspberrian)
As the other post said, you must here also do a search and replace to replace all occurences of "/home/demouser" with "$HOME"
In step 6, you will now probably get a java Exec format error. That seems to be because the installer installed a gentoo 64-bit version of java instead of a 32-bit version for raspberrian. I fixed it by doing the following:
sudo apt install openjdk-8-jdk openjdk-8-jre
The correct java then seemed to be installed on /usr/lib/jvm/java-8-openjdk-armhf/bin/java
Now, you must open runMC1_14_4_OptifineF3.sh in your favourite editor again.
Enter /usr/lib/jvm/java-8-openjdk-armhf/bin/java just before the old java path before "-Xmx2G"
So that's it then. NOOO you said?? No you get an error message complaining about "Failed to locate library: liblwjgl.so"?
After a lot of googling, it seems that the arm64-version of liblwjgl3 that was downloaded is not compatible with 32-bit raspberrian. Finally I fount out from viewtopic.php?t=218742
that I could download compatible files from https://www.dropbox.com/s/y6gaickl7daob ... ar.xz?dl=0
Download these files, and place them in a new directory /home/pi/lwjgl3arm32/
Then, again open runMC1_14_4_OptifineF3.sh yet again. This time, you must do a search and replace, replacing all occurences of $HOME/lwjgl3arm64/ with $HOME/lwjgl3arm32/
So now finally the game is up and running. Wait, it is not?
Oh, the lwjgl-files we downloaded seems to just include the system-specific so-files, and not the jar-files. We can then just do the following:
cp $HOME/lwjgl3arm64/*.jar $HOME/lwjgl3arm32/
And finally, the game actually starts up, and is even playable (I reduced render chunks etc in options)
Hopefully you got this far too