As for german:
Joseph Salmons has always been struck by the pervasiveness of the argument. In his visits across Wisconsin, in many newspaper letters to the editor, and in the national debates raging over modern immigration, he encounters the same refrain:
news.wisc.edu
"Not only did many early immigrants not feel compelled out of practicality to learn English quickly upon arriving in America, they appeared to live and thrive for decades while speaking exclusively German."
Second and third generation
"Some second- and even third-generation German immigrants who were born in Wisconsin were still monolingual in German as adults.
“These folks were committed Americans,” says Salmons. “They participated in politics, in the economy, and were leaders in their churches and their schools. They just happened not to conduct much of their life in English.”"
Percentages
"In 1910, the researchers still found robust populations of German-only speakers in these communities. The census identified 24 percent German-only speakers in Hustisford, 22 percent in Schleswig (Manitowoc County), 21 percent in Hamburg and 18 percent in Kiel."
German in government coorespondence.
" The researchers found correspondence in the 1890s from school districts to the office of the state school superintendent that were written entirely in German. This is after the Bennett law of 1889 that required schools to be taught in English."
For general trends. This only goes up to 2010, but other data suggests the percentage of English speakers continue to rise.