Good evening Philipp and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
Before we begin, I highly recommend reading the 3 following articles on translation:
Simply put, this might be a simple case of some language files requiring an update (so that you can see the translatable strings)
However, in your specific case, since you are interested in customizing the Add Event Page, I believe that the best solution would be to take advantage of our template override system.
You might want to read our Themer’s guide to get a sense of how that works.
Basically, you can edit the Add Event view page to your liking and upload it back in the /tribe-events/ folder of your WordPress theme.
Let me know if that helps.
Have a great day!
Geoff B.