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.
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.
