Hi,
my father is French and my mother is English. I was brought up in England. When the dominant culture is English, your friends, TV, books, films, you won't really be bilingual without a lot of effort with the other language, in my case, French. I went to France on holiday a lot as a child, so my French isn't bad. But language acquisition needs motivation and it is hard to be motivated to learn French if you are surrounded by English. My sister lives in France with her French husband and two sons and she has made a concerted effort to talk to them in English in a French-dominated culture. This means times when she only questions and answers them in English. Result: they are truly bilingual after a lot of effort.
Being bilingual is a good thing. Not only do you know two languages, you are aware of the structure of language itself, the metalanguage, which gives you a key into other languages. My nephew was talking to my Dad on the phone when he was about 7. He wanted to describe the weather in Normandy. There is a biting wind called 'La Bise', so he said, 'The wind is bising'. Brilliant. As far as I know there is no correlation between facility with languages and maths. My dad was a languages teacher for 30 years and is really good at maths.
Language, like any branch of learning is about application and use. If you have had more than one job, you find ways to apply the knowledge gained from one to use in the other. Similarly with language, besides the practical benefits of social and business communication with other cultures, it expands your knowledge base and your thinking skills. We use so little of our brain and this is great brain training.
So my advice is be systematic about it. Don't just assume it will just happen by osmosis - there needs to be motivation on behalf of the learner. And it must be fun and enjoyable.
By the way. I am learning Chinese. And I'm pretty damn motivated:icon_cheesygrin: