As a native English speaker there are words there that I've never seen and others were I would not know the original adjective. In wiktionary, you can click them see etymology, here https://en.wiktionary.org/wiki/-th#English

For example, apparently "filth" comes from the equivalent of "foul".

> As a foreigner I’m curious what the original adjective was for filth, length, ruth, wealthy, wrath

Looked the ones I didn't know the adjectives of up on wiktionary: foul, long, rue, possibly the archaic "wele" (but there are multiple etymological paths), and basically "wrath" but pronounced slightly differently going back thousands of years.

Wealthy's root form is weal. As in we should do things for the common weal of the people.

Weal is defined as happiness, well-being, prosperity, or that which is best for someone.

Earthy means someone with big ears. Or lots of ears, maybe.

I am a little surprised that ruth is spelled that way instead of like rueth, which seems more like how I would expect it to be said.

length <- long

strength <- strong