Minecraft only loads language files when the entire name is lowercase, meaning en_us.json will load, but en_US.json wont, this is how you get translations being missing with seemingly no issues.
This mod fixes this by fixing their names by looking for misnamed files and adds them to lang/*.json
This is a bandaid fix, and we don't encourage doing things wrong, this project exists so those overlooks still work, but they should still be fixed by their developers.
This mod Requires