The constraint message that is displayed when a required language couldn't be found in the labels of an entity has languages names written in the same language that is required instead of in the language of the user interface. When this is a language I don't understand I'm unable to know which language is the one missing. I can't fix the problem without understanding the language, but it would be nice knowing at least which one the missing language is.
Example:
https://www.wikidata.org/wiki/Q100257812#P4529
What happens?:
Language names in wbqc-violation-message-label-lacking message are displayed written in that language.
What should have happened instead?:
Language names of the message should be displayed written in the language of the user interface.