Plex is not a file browser, which simply shows you all your files in the same folder structure as you have them stored.
Instead, it uses the folder structure and the file names to recognize the titles of your series and movies and match them to external metadata. All of this is stored in its internal database.
Because of this, and how this database is structured, there are consequences for the user.
One of these consequences is: Each show can only exist once in a Plex library.
You cannot add the same show in different language versions to the same Plex library.
Doing this would require support for Editions in TV Show libraries, which are currently only available in “Movie” libraries.
For now, you’d need to create different libraries. You could use one to collect all the English stuff and the other for all the German stuff.
Then you can pick a separate language for all the metadata (descriptions, titles, posters etc.) within a library.
(tip: for single shows, you can define a deviating metadata language in the properties of that show)
A better solution would be to have both languages within the same file. Then the audio and subtitle tracks are chosen automatically, based on each user’s language preferences. This of course requires that all audio and subtitle tracks can be synced to the one single video track.
Another downside to this is that you have to decide on one language for the metadata.
If the majority of your Plex users demand German media, you should probably pick German for the metadata as well. But you can set your own account language preferences that you will automatically get the English audio, if it’s available.
P.S. your description of the automatic capitalization of titles sounds like you are using a generic DLNA-compatible player to access your Plex server.
Just don’t. The DLNA code in Plex is decades old by now and is limping barely along. It doesn’t look like it will get a major update any time soon.
Use one of the officially supported Plex clients instead https://support.plex.tv/articles/categories/player-apps-platforms/
