However if I visit a subpage url with the same slug for both languages (for example a perosnal name that doesn’t get translated) without a language slug it always forwards me to the default language version of the subpage:
would make sense, wouldn’t it? It is an edge case because a lot of slugs can be translated and translated slugs do get redirected to the correct language.
If you want to redirect all pages to their equivalent in the detected languages, you can achieve this with routes. Kirby doesn’t do this automatically.