As for knowing it fluently, my Mum and various of her friends are all Dutch living in the UK. Many of them earned some money from it in some way, including:
- doing translations
- getting jobs in the local port because they were bilingual
- teaching night classes in the language
- emergency translator for customs
And that's Dutch, where even 20 years ago a lot of them spoke English fluently. It can be useful.My friend's wife is a US Bulgarian who speaks French and got a grant from her uni to go to Bulgaria this summer to interview Bulgarian software companies (I think including Chaos/XCom maker Julian Gollop who lives there now).
Then again I know others who never use their 2nd or 3rd language.
Basically, it opens doors to experiences if you want to leverage it.