What languages were spoken by Plateau Indians?

Answer 1

The so-called "Plateau tribes" did not all speak the same language:

• the Kootenai spoke a Kitunahan language perhaps distantly related to Algonquian
• the Flatheads spoke a Salish language (the most easterly of all Salish tribes)
• the Pend d'Oreille spoke a Salish language
• the Shuswap people spoke a Salish language
• the Lillooet tribe spoke a Salish language
• the Thompson tribe spoke a Salish language
• the original Nicola people spoke an Athapaskan language
• the Okanagan tribe spoke a Salish language
• the Lakes tribe spoke a Salish language
• the Colville tribe spoke a Salish language
• the Kalispel spoke a Salish language
• the Wenatchee spoke a Salish language
• the Chelan spoke a Salish language
• the Sanpoil people spoke a Salish language
• the Nez Perce people spoke a Shahaptin language
• the Coeur d'Alene tribe spoke a Salish language
• the Cayuse people spoke a Penutian language
• the Umatilla people spoke a Shahaptin language
• the Walla Walla tribe spoke a Shahaptin language
• the Palouse people spoke a Shahaptin language
• the Warm Springs /Tenino spoke a Shahaptin language
• the Yakima people spoke a Shahaptin language
• the Wasco people spoke a Chinookan language
• the Wishram people spoke a Chinookan language
• the Molala people spoke a Penutian language
• the Klamath people spoke a Penutian language
• the Modoc people spoke a Penutian language

Although many of these languages are related they are not always mutually understandable; Yakima Shahaptin is not the same as Nez Perce Shahaptin for example.

Answer 2

Athabascan Tsimshian Chinook and many more
They speak Athapaskan, Salishan and kutenai