Thanks, I do understand how i18n works for plugins. I should have asked if there is a way to do this without having to fork or modify the existing plugin. It seems like the answer is no.
Personally, I think this is a lacking feature of NodeBB.
There is this plugin: https://github.com/psychobunny/nodebb-plugin-badges
But, I think @psychobunny stopped developing it a while ago, so it probably isn't compatible.
Thanks a lot!