@Spinster Wouldn't that be a bit too limiting, potentially for the future, if any new properties are minted? I'm not sure myself so just asking about the full context here. My thoughts are that maybe allowing options for further constraints in this dialog like properties that are subclassed under https://www.wikidata.org/wiki/Q18616084 or https://www.wikidata.org/wiki/Q20824104 ? Maybe there could be an extra toggle filter in this dialog that says "Only languages" and it filters to only those properties under those 2 subclasses?

Oh you likely mean only the Wikimedia language code https://www.wikidata.org/wiki/Property:P424 ?

File captions on Commons are not properties. They are technically the equivalent of Labels on Wikidata.

I just checked, and retrieving Labels via data extension from the Wikidata reconciliation service behaves the same, it's also a bit confusing there :-)

Ah, ok