Earlier quoted context omitted.
I was having a related discussion with my wife about why our nephews weren’t taught Spanish as a first language (we live in Texas, their dad is bilingual). Frankly, having English as your first language gives you a much higher advantage in life in the current zeitgeist. There’s no incentive to learn RandomLanguage first, but there is every incentive to learn English first.
Nope, learning multiple languages from the beginning has been shown to have many benefits. I speak both my native language and English at native levels, simply because I learned my language at home and English in school. Only teaching English is really a waste when you live in an English speaking country, as the child will automatically pick up English through school and friends.
More to the point: Speaking multiple languages expands your ability to express and describe everything, which leads to having more variety in your lines of thought. Different languages have different ways of thinking, thus speaking multiple languages nurtures an open mind.
Source: Me, native English and Japanese speaker.