While I am reading something written in English, my mind wants to stick to that language and makes me refrain from taking up books written in other languages such as German and Japanese. I feel the existence of the law of inertia in my mind.
My mind is now in English mode, although I am not a native. It tries to reject any input that is not written or spoken in English. Even if my wife, whose name escapes me at the moment, happens to speak to me in Japanese, I will respond in English.
Reading English textbooks for high-school students
When I was young, most of the students in Japan started learning English at the age of twelve. I should have kept the textbooks that I used in junior-high school and high school, but I threw them away. When I was about forty years old, I followed Masao Kunihiro’s advice “Repeating Reading aloud(只管朗読)”; I got English textbooks for high-school students that were used as authorized textbooks in Japanese schools. The title of the books I bought was “The Crown English Series.”
In book II, which is, I suppose, written for sixteen-year-old students, you can read passages from “I Believe” by Bertrand Russell, some from “Momo” by Michael Ende, some from “Japanese and Americans” by Edward Seidensticker, etc.