I'm not sure if Meta Box strings are translated to your language 100%, you can check the untranslated strings here https://translate.wordpress.org/projects/wp-plugins/meta-box/
If the string is not translated, it will display in the English language. If it does not work, can you please share some screenshots of the issue?
It seems you did not understand what I was trying to tell you, so I will elaborate:
The string "+ Add more" is correctly translated to my sites' language (German), but it should not since my user profile language is English, which is different from my sites' language. This results in a mixed language user interface. My concern is that you should honor the users language settings over the sites' language.