I found some enchantments that don't have a proper description, and I want to add some but I can't find the correct file.

It's recommended to ask the mod developer to add the descriptions themselves so that everyone can have the descriptions. If you want custom descriptions you can add your own using a resource pack with a custom language file. The example is here.

Do I need to open the mod file to find the langauge to change or add descriptions?

No, you use a resource pack.