I am quite fluent in 5 European languages (French, Italian, Spanish, Portuguese and English) and less so in German. Six months ago I started learning Sango, which is the native tongue of the Republic of Central Africa. With regular practice and a real desire to have a genuine communication with other people and cultures, everything comes in.
The sadder note is, however, that if you don't publish in an international language, you're almost invisible: English, or perish ...
Thanks for sharing your experience, Frederic Lefrancois . We’re similar then. I’m also fluent in 5 European languages (Portuguese, my native tongue, English, French, Spanish and German). I took courses and also studied a lot by myself using selfteaching materials. Nowadays, it still takes a lot of time and effort to both maintain and advance my level of fluency in those languages.
If you were not born in a bilingual/bidialectal society, and if you want to learn a new language, then take some language courses in a university/institute. In the same time, you can study on your own in order to learn the language more rapidly. Note that it is important to interact with native speakers of the foreign language; in order to achieve this, you might also live for a while in the foreign country. However, if you are born in a multilingual society, you can easily become a native speaker of the languages spoken there.