You do know that Montana was part of Mexico up until we stole that land from them right? The name Montana comes from the Spanish word Montaña, which in turn comes from the Latin word Montanea, meaning "mountain", or more broadly, "mountainous country". I can't imagine why there would be Spanish speakers in a place that has spoken Spanish for over 300 years.
I think someone needs to study up on the history of this damn country. Throughout the Mississippi and Missouri River valleys, you are going to find people that speak French, French Creole, and Spanish as well. You will hear German spoken in certain parts of this country, mostly the upper Midwest and parts of Pennsylvania. In Minnesota, you will hear people speaking Nordic languages.
Anywhere west of the western boundary of Louisiana Purchase territory was all Spanish Territory before the Mexican-American War that was triggered by white settlers in the Mexican state of Tejas declaring themselves a Republic and then they begged the US government for help when they were losing the war they started.