[NonZeroSum]

I am not sure the bi-lingual hypothesis has been demonstrated to be true. I can think of several people who know 2, 3 or even 4 languages but as adults have not been able to extend understanding to a 5th or 6th language.

It's true statistically, as far as I know. Your anecdotes wouldn't change that.

[muawiyah]

Frankly I think the supposed bi-lingual advantage is just European propaganda designed to encourage transborder comity. As adults only a few people retain the ability to learn a different language.