In Linguistics, a language is considered dead when it has fewer than 50 speakers. Some say it might be dead even with 500. Virtually all of the Native languages in the USA--numbering some 300 at colonization--are endangered to some degree, most considered severely endangered. While many exist still, few will survive without concerted effort.
Even Navajo, with an estimated 120,000, is considered at risk. The loss of any language means the loss of its culture. As speakers switched to English, concepts & beliefs & systems of categorization etc are all lost. Given that each culture is responsive to its geographic and social conditions, we can't afford to lose ANY language.
Sioux/Lakota has approximately 2000 native speakers worldwide. Fewer than 2% speak the language natively. It's severely endangered. ERROR! and correction follows....
"It is related to, and its speakers interact with, Kickapoo, which is also severely endangered." It's Meskwaki that is related to Kickapoo. Souix is a group of related varieties, one of which is Lakota, which has several varieties as well.
Oneida is endangered with perhaps 220 native speakers--world wide.
Comanche has fewer than 100 speakers left....all over the age of 50.
Blackfoot has perhaps 3300 native speakers, and is considered severely endangered.
Other commonly known languages that are severely endangered include Arapahoe, Cherokee, Creek, Crow, Hopi, O'odham, Shoshoni, Ute, Western Apache, and Zuni.
There is some good news, however. Even as extinction nears, many efforts are underway to revitalize languages. Lakota, Meskwaki, Ojibwe, and others are introducing language programs. Such programs MUST include the children; in fact, many begin there. Early education immersion programs are vital.
To succeed, native speakers are needed, materials in the language are needed, and support before/after school is needed. And so programs are also introduced for adults. The Meskwaki run a language camp in the summer for all ages to learn and practice together.
A notable program is Wampanoag, spoken in Delaware. The language died--it was considered dead because it had no native speakers. For 100 years the language had no native speakers. Then Jesse Little Bird had a vision...to bring back the language. There were hundreds of documents written in Wampanoag, treaties, the Bible, and so on. A daunting task, to revive a language solely from written documents. How, for example, to know how something is pronounced?
Jesse Little Bird was determined.....went to MIT, talked to Linguistics and Applied Linguistics professors (including Noam Chomsky). She found tremendous support and began taking classes in phonology, morphology, syntax, semantics, and so on. One aspect of her learning was that there is a process for determining (or making a really good guess) how to pronounce a word. The word is compared to similar words in related languages, and the history of sound changes suggests what direction the word might go. So now it was possible to have a phonology of Wampoanog as well as a syntax (grammar).
Her community supported her. A dictionary was begun, materials in Wampanoag created. And Jesse began speaking the language to her daughter--the first native speaker of the language in over 100 years. Programs for children and adults developed and the community comes together to learn and study.
And now there are perhaps 5 children learning the language natively, And now the language is no longer dead, it is called Awakening. Isn't that terrific? The film documenting all this is called We Still Live Here and is well worth watching (http://www.pbs.org/independentlens/films/we-still-live-here/).
There are other resources for language revitalization, perhaps unrecognized. Children who were sufficiently exposed to the language, even if they don't speak it now, still have a receptive understanding of it. With sustained input before the age of 5, or 7, such a child can activate their receptive knowledge through renewed contact and input. Even up to the age of 10 or so, a child will have some residual knowledge of the language. Any child in this situation should seek out the chance to revitalize their own language, and then contribute to the revitalization of others.
And we should all encourage parents to share their languages with their children. They won't get confused. There is room for more than one language (or two or three). They will develop a greater understanding of different world views. They will in fact know English better. Languages are precious resources; let's keep them alive.
*Most of the statistics come from the Endangered Languages Project website http://www.endangeredlanguages.com.
Note: US government policy was, predictably, all wrong. Benign neglect is one way to encourage language loss. People and communities gradually shift to the more "useful" language. The boarding schools and attempts at forcible assimilation actually had the opposite effect of awakening a desire to maintain languages.