I have no clue what to do about it as I'm new to modding.
The YouTube tutorial:
The GitHub link: https://github.com/bananapizzuh/Bananafoods/tree/bananapizzuh-bananafoods-forge-1.12.2
Edit:
I'm using IntelliJ IDEA 2020.
After building the mod and running it on the minecraft launcher the .Lang file started to work.