The language options are based on responses from the ACS (American Community Survey), a yearly survey done by the Census Bureau. They go around to different households interviewing people about what languages they speak at home and respondents give a variety of answers, which I believe they try to categorize after the fact. Some answers are certainly families of languages. I believe in this case that option refers to this indigenous language of the Sioux: https://en.wikipedia.org/wiki/Dakota_language.

Here's more links to the dataset I used if you're interested: https://github.com/pcaisse/language-map-us#data

Yeah, it’s one of three, but not the most popular of the bunch. Lakota is much better known. If you are grouping them then you probably want to change the label.

Nice site.