ISO 639-1 to you, you can expect specific values to be associated with them. There is a list of constraints expected on the talk page. For the constraints mentioned, a list with constraint violations is published.
The point of such a list is to find members of the Wikidata community to cooperate and fix those errors.
One constraint used on many properties is associated with the deprecated property P107, the "main type (GND)". Many a list of constraint violations are regularly generated like this one for the P218.
When asked, it was NOT thought to be a good idea to remove all the erroneous constraint reports that are based on a false premise. So far I have constrained myself from uttering profanities. It is better to make the case and make the case publicly and loudly.