“There are too often parts of our country … where English is not spoken by some people as their first language.”
“And that needs to be changed,” he said.
The most important priority for immigrants should be “to be and to feel British,” he said, “and to learn English.”
To me, an English politician saying, "everyone in England should speak English" is saying something quite obvious and popular. To be clear, Boris Johnson is an idiot and will be a terrible PM. But this issue is both a winner with voters, and learning the dominant language of the country where you live is simply a good idea. This stance is both good policy and broadly popular.
As a successful immigrant to the US has said,
First, learn the English language and blend into the American culture, even as you honor and remain proud of your own heritage. I don't say learn the language out of any sense of etiquette or duty. Do it so you can participate fully in the life of the nation and make the most of living in this country.
I came here in 1968 speaking only a little English. To make it in business and Hollywood, I knew I had to take English lessons, speech lessons, accent-removal lessons—anything to improve my chances of success. And I happily did it all.