New property https://www.wikidata.org/wiki/Property:P10609 is also broken. The formatter URL is https://bioportal.bioontology.org/ontologies/PLOSTHES/?p=classes&conceptid=http%253A%252F%252Flocalhost%252Fplosthes.$1 and the values of $1 include a # symbol, e.g. 2017-1#9401. The link doesn't work. This direct link does work: https://bioportal.bioontology.org/ontologies/PLOSTHES?p=classes&conceptid=http%3A%2F%2Flocalhost%2Fplosthes.2017-1%239401

So when will this language code be active and usable in Wikidata? I just tried to use it and it was not recognized.

The immediate use will be to add the Carolinian language form of official name to the Wikidata item for Northern Mariana Islands. However, I imagine it will be used for other items related to Northern Mariana Islands that have a Carolinian name for the item. That could include individual islands in the Northern Marianas, names of persons from that place, corporate bodies, works with a title in Carolinian, and websites whose language is Carolinian. Of course, it could potentially be used for anything that has a name in Carolinian.