Dalecarlian is recognised by Ethnologue. And according to the information provided, some 1500 people speak it. It is the first time that I did look up a record in Ethnologue to find that it does not exist in the ISO-639-3.
I looked it up because someone volunteers to localise MediaWiki in Betawiki. The question is, what to do with such a request. The first thing to realise is that the dlc code is unlikely to be ever used as an ISO-639-3 code. The second thing is that it IS a living linguistic entity and, it will be not too hard to argue that if it is not a lanugage, it is at least a dialect.
So allowing for the localisation of Dalecarlian is not problematic. From a standards point of view, there is only one issue to be resolved. What is the correct code that we should use for it?