[killedbydeath]

It's not obvious that speaking second language better will come out at the expense of English. I grew up bilingual and was at the top of my class for my second language compared to monolingual natives. There are whole countries of bilingual/trilingual people in Europe who speak their main language very well still. I think knowing a second (and later third) language gave me a better understanding of language structure. Same in sports, if you are athletic, you will have easier time picking up new sports.

[usrusr]

My pet theory is that multilingualism multiplies the delta in natural talent: gifted people become stellar writers/talkers, whereas less talented people might suffer from career-inhibiting deficiencies in all languages and often would have been better off focusing on a single language.

I'm far from considering myself multilingual, but if I could somehow trade a part of my English skill for a magical boost in my native language I would gladly take it: half of my English abilities would still be plenty for understanding the occasional stackoverflow post, this place here would be just fine without my contributions and ordering food at a restaurant is actually easier when you can fully embrace the "helpless tourist" level. In my natural language, one level better might be the deciding difference between writing as a necessity and writing as a selling point.

[hkmurakami]

To a first order, it's about how much time you invest into each skill. For me, a non Latin language didn't have noticeable spillover effects into English until much later in my life when I developed decently advanced linguistic proficiency in both languages.

This isn't to say I didn't do well in school -- I was at the top of my class in high school and went to the top ranked university in the country. Still, had I invested more time trading and writing English material during my childhood and adolescence, those skills would have developed much earlier.

And I agree with you about sports. I'm an advanced or white level athlete in 4 or 5 sorts, and it's very easy for me to become producer at a new sport, and I often notice body control, posture, focus, or valance related carryovers.