I've been bathed in french and english and live in a flemish speaking town. While french and english are natural, flemish is a constant challenge because it's my job's language and have only heard it several hours par week at school. But it was enough to feel the difference with other french speaking colleagues from Brussels during the time I was working there. Their flemish was not as fluent.
Being bilingual or trilingual often isn't enough....I have to react quickly (on the phone for instance), and this means to think in that other language (really not easy).
Sometimes, another language is motivating: at the Clubmed, german girls could really be quite attractive! German is our official 3rd language and the 4th learned at school..Luckily, german has a lot of similarities with dutch (flemish).