Modal windows switching and mixing languages

Hello!

When I first started my Mac Book Pro 2017, I chose German as my first language and English as my second language. I also did so in the Language and Regions settings. Although I chose German, I noticed that sometimes the OS (macOS High Sierra, 11.13.3) is mixing up languages (German & English) in modal windows. This only occurs in OS-related outputs (for example, Finder), not in applications. (Please see attached screenshot.)

Is this a bug?

Despite this being a first-world problem, it's annoying because I paid almost 3.000 Euros for my Mac Book, so I want it to work properly.

Thanks!