Those the argue that immigrants must assimilate by speaking English have no clear understanding of history. More Americans speak Spanish than the entire population of Spain. As the United States expanded westward it absorbed the existing culture and language. The Treaty of Guadalupe Hidalgo, that facilitated the largest population absorption of people, did not dictate an English only rule nor the eradication of the Mexican culture or the Spanish language.
As they say, a picture speaks a thousand words:
Now that you understand the historical context of Spanish in America or the Mexican culture within the country, stop demanding that immigrants learn to speak English to assimilate into America.
How about learning Spanish, instead?