Certain addons add back aspects from Thaumcraft 4, and a couple of them lack a description:

Oddly enough I wasn't easily able to pinpoint which addon does this, and I'm not sure where would be the best place to add the description. I thought about potentially overriding Thaumcraft's language file?

Would appreciate any advice, thanks!

The aspect description is a base Thaumcraft feature - every aspect has a description with a lang key of tc.aspect.<tag>. Due to how language keys work, you can add them anywhere as long as the resource loader can find them - it doesn't have to be in Thaumcraft's lang file itself. Ideally the addons would add it in their lang file, but you can also make your own.

Thank you :)