The cldr extension includes a nonstandard dlc language code; @Nikki investigated where it came from:

In T341409#9339684, @Nikki wrote:

In T341409#9337791, @Lucas_Werkmeister_WMDE wrote:

The only one I can’t make any sense of is dlc, which MediaWiki says is “Dalecarlian”, yet Dalecarlian language doesn’t mention a dlc language code. Apparently the cldr extension (but not the actual CLDR) took it from Ethnologue in 2008.

I think this one should be removed from the CLDR extension. It's not a valid code and when I checked the other day, I wasn't able to find any current use of that code, nor anyone requesting it in the first place. It was just suddenly added in https://github.com/wikimedia/mediawiki-extensions-cldr/commit/45e8e42c040a5be96f380d48aea52819db7f1c7e with no explanation.

Some history of the dlc code:

Before ISO 639-3 was a thing, Ethnologue used its own set of codes (using uppercase letters). The 14th edition (2000) had an entry for Dalecarlian under the code DLC (link). For the 15th edition (2005), they switched to using codes from the draft ISO 639-3 standard (using lowercase letters). The 15th edition had an entry for dlc, saying that was also the ISO 639-3 code (link). ISO 639-3 was published in 2007 and did not include an entry for dlc or "Dalecarlian". The 16th edition (2009) did not include dlc any more (link). ISO 639-3 codes were not added to the IETF/IANA/BCP 47 subtag registry until much later, in 2009, so it didn't get included there either.

I assume dlc was in the draft version of ISO 639-3 and was removed before it was officially published, and then was removed from Ethnologue too, but it had already been added to LocalNamesEn.php by then.