Hey Sven,
Sorry about these missing translation string! Thank you for altering us to them – hopefully we will fix that in the coming releases.
As for editing the files, you do risk losing those files in a plugin update. You can move the customized files into your theme if you follow the guidelines in our Themer’s Guide here → https://theeventscalendar.com/knowledgebase/themers-guide/
However, those changes would be overwritten in a theme update – so you unfortunately cannot be 100% safe 🙁
As long as you just make a backup of your customized files somewhere on your computer, you should be able to replace your customizations without issue if they are ever overwritten in a theme or plugin update.
I hope this helps!
– George