Fix translation slug relying on identical folder names

Currently, translations only work, if the translated pages are located in a folder with the same name. However the folder name might also need to be translated, so we should make this possible.

E.g. currently we need to do this:
DE: https://myfabcity.com/de/neuigkeiten/mein-post.mdx
EN: https://myfabcity.com/de/neuigkeiten/my-post.mdx

But we want to allow this:
DE: https://myfabcity.com/de/neuigkeiten/mein-post.mdx
EN: https://myfabcity.com/de/news/my-post.mdx