@DarkLord76865 said:

In the site for it there are English and Croatian languages added, the English has no additional information, only a short one sentence description, there isn't even a need for English translation to exist, but I guess it must?

Translation pages can be created for any language as users can add overviews in any language.

The problem I have is with episode names, I set their original names in Croatian, but I didn't set English names because they don't exist. TMDB set English name for each episode to "Episode #id" and even if I delete it, it still comes back.

If the series have not been broadcasted in an English speaking country, this is the correct way to do.

I wouldn't really mind, but it causes Jellyfin to not work properly since it pulls these English names which shouldn't even exist instead of original episode names that are set in Croatian.

If you want that your scraper take the Croatian episode titles, you will have to configure it to take them. If you don't know how to do it, you will have to contact their support team to get help.

What should I do because episode names of style "Episode #id" definitely aren't real episode names.

Configure your scraper so it take the Croatian titles.

Should I set the English episode name to the same name as the original Croatian name?

No.