Title: Sioux language
Content:  Sioux is a Siouan language spoken by over 30,000 Sioux in the United States and Canada, making it the fifth most spoken indigenous language in the United States or Canada, behind Navajo, Cree, Inuit languages, and Ojibwe. Since 2019, "the language of the Great Sioux Nation, comprisedof [sic] three dialects, Dakota, Lakota, and Nakota" is the official indigenous language of South Dakota.
Title: Sioux
Content:  The Sioux or Oceti Sakowin (Dakota: Očhéthi Šakówiŋ /otʃʰeːtʰi ʃakoːwĩ/) are groups of Native American tribes and First Nations peoples in North America. The modern Sioux consist of two major divisions based on language divisions: the Dakota and Lakota; collectively they are known as the Očhéthi Šakówiŋ ("Seven Council Fires"). The term "Sioux" is an exonym created from a French transcription of the Ojibwe term "Nadouessioux", and can refer to any ethnic group within the Great Sioux Nation or to any of the nation's many language dialects. Before the 17th century, the Santee Dakota (Isáŋyathi; "Knife" also known as the Eastern Dakota) lived around Lake Superior with territories in 
Title: Great Sioux Nation
Content:  The Assiniboine (Nakota), who live to the North of most of the Sioux peoples, speak a Siouan language closely related to the Sioux language but are not considered part of the modern Oceti Sakowin. The Assiniboine were the first of the Siouan-speaking peoples to leave the Eastern Woodlands and move onto the Great Plains. They had developed as a separate people sometime before 1640, when they were first documented as a tribe by French missionaries. Because of this, other Sioux-speakers referred to them as the Hohe, or "rebels". The Canadian Nakoda (Stoney) possibly began as the westernmost band of the Assiniboine, but were referred to as a separate people after about 1744. Later, in the nineteenth century, further bands belonging to the Dakota peoples and to a lesser extent to the Lakotas, also moved to Canada and their descendants still live in several of the Canadian First Nations.
Title: Great Sioux Nation
Content:  The Great Sioux Nation is the traditional political structure of the Sioux in North America. The peoples who speak the Sioux language are considered to be members of the Oceti Sakowin (Očhéthi Šakówiŋ, pronounced ) or Seven Council Fires. The seven-member communities are sometimes grouped into three regional/dialect sub-groups (Lakota, Western Dakota, and Eastern Dakota), but these mid-level identities are not politically institutionalized. The seven communities are all individual members of the historic confederacy. In contemporary culture, the designation is primarily a linguistic, cultural, and for some, political grouping. Since 2019, Sioux language has been an official language of South Dakota.
Title: Lower Brule Indian Reservation
Content:  The Sioux consist of a group of self-governing tribes speaking one of three dialects of the Siouan language: Dakota, Nakota and Lakota. The Dakota or Santee, who identify as by the autonymns of Mdewakantonwan, Wahpetowan, Wahpekute, or Sisseton, range in territory from the Ohio River valley to South Dakota. The Dakota or Nakota, known as the Ihanktonwan/Yankton or Yanktonai/Ihanktonwanna, range from eastern Minnesota to the Missouri River valley. The Lakota, or Western Teton/Tituwan Sioux, consisting of the Oglala, Mniconjou, Sicangu, Sihasapa, Oohenunpa, Hunkpapa, and Itazipco, traditionally ranged from lands east of the Missouri River valley to the Rocky Mountains. A common history and language, a strong respect for the land and nature, the common use of Pipestone and the reverence held for the stone, and ceremonies such as the Sun Dance, Sweat lodge, and Vision Quest, bind these peoples together. The 
Title: Lakota language
Content:  Lakota (Lakȟótiyapi), also referred to as Lakhota, Teton or Teton Sioux, is a Siouan language spoken by the Lakota people of the Sioux tribes. Lakota is mutually intelligible with the two dialects of the Dakota language, especially Western Dakota, and is one of the three major varieties of the Sioux language. Speakers of the Lakota language make up one of the largest Native American language speech communities in the United States, with approximately 2,000 speakers, who live mostly in the northern plains states of North Dakota and South Dakota. Many communities have immersion programs for both children and adults. The language was first put into written form by European-American missionaries around 1840. The orthography has since evolved to reflect contemporary needs and usage.
Title: Sioux
Content:  Sioux meaning "little snakes" (compare nadowe "big snakes", used for the Iroquois). The spelling in -x is due to the French plural marker. The Proto-Algonquian form *na·towe·wa, meaning "Northern Iroquoian", has reflexes in several daughter languages that refer to a small rattlesnake (massasauga, Sistrurus). An alternative explanation is derivation from an (Algonquian) exonym  na·towe·ssiw (plural  na·towe·ssiwak), from a verb  *-a·towe· meaning "to speak a foreign language". The current Ojibwe term for the Sioux and related groups is Bwaanag (singular Bwaan), meaning "roasters". Presumably, this refers to the style of cooking the Sioux used in the past. In recent times, some of the tribes have formally or informally reclaimed traditional names: the Rosebud Sioux Tribe is also known as the Sičháŋǧu Oyáte, and the Oglala often use the name Oglála Lakȟóta Oyáte, rather than the English Oglala Sioux Tribe or OST. The alternative English spelling of Ogallala is considered improper.
Title: Sioux
Content:  The Sioux people refer to the Great Sioux Nation as the Očhéthi Šakówiŋ (pronounced, meaning "Seven Council Fires"). Each fire is a symbol of an oyate (people or nation). Today the seven nations that comprise the Očhéthi Šakówiŋ are the Thítȟuŋwaŋ (also known collectively as the Teton or Lakota), Bdewákaŋthuŋwaŋ, Waȟpéthuŋwaŋ, Waȟpékhute, and Sisíthuŋwaŋ (also known collectively as the Santee or Eastern Dakota) and Iháŋkthuŋwaŋ and Iháŋkthuŋwaŋna (also known collectively as the Yankton/Yanktonai or Western Dakota). They are also referred to as the Lakota or Dakota as based upon dialect differences. In any of the dialects, Lakota or Dakota translates to mean "friend" or "ally" referring to the alliances between the bands. The name "Sioux" was adopted in English by the 1760s from French. It is abbreviated from Nadouessioux,  first attested by Jean Nicolet in 1640. The name is sometimes said to be derived from an Ojibwe exonym for 
Title: Western Siouan languages
Content: Mandan † ; Nuptare ; Nuetare Missouri River Siouan (a.k.a. Crow–Hidatsa) ; Crow (a.k.a. Absaroka, Apsaroka, Apsaalooke, Upsaroka) – 3,500 speakers ; Hidatsa (a.k.a. Gros Ventre, Minitari, Minnetaree) – 200 speakers ; Mississippi Valley Siouan (a.k.a. Central Siouan) ; Mitchigamea? † ; Dakotan (a.k.a. Sioux–Assiniboine–Stoney) ; Sioux – 25,000 speakers ; Lakota – 2,100 speakers ; Dakota (sometimes classified as Western and Eastern Dakota) – 290 speakers ; Assiniboine – 150 speakers ; Stoney – 3,200 speakers ; Chiwere-Winnebago ; Chiwere † ; Winnebago – 250 speakers ; Dhegihan ; Omaha–Ponca – 85 speakers ; Kansa-Osage ; Kansa † ; Osage †, on ongoing revival ; Quapaw – 1 speaker ; Ohio Valley Siouan ; Virginia Siouan ; Tutelo † ; Moneton † ; Mississippi Siouan ; Biloxi † ; Ofo † The Siouan family proper consists of some 18 languages and various dialects:  (†) – Extinct language Another view of both the Dakotan and Mississippi Valley branches is to represent them as dialect continuums. All the Virginia Siouan dialects listed here are thought to have been closely related to one another; the term Tutelo language is also used in reference to their common tongue.
Title: Swinomish Indians of the Swinomish Reservation of Washington
Content:  English is commonly spoken by Tribal members. Some elders speak Swinomish (also known as Skagit), a Lushootseed language that is part of the Central Salish language family. Some also spoke Samish, another Central Salish language.

Question: what language did the sioux indians speak?
Answer: Sioux language.