> A solution could be to make 3 mirrors (NL, FR and
> EN) and put them in a different folder according to
> the language.
> EN for the English one, FR for the French one and so
> on.
> Only problem is that I don't really know how to do
> this since the file names of the pages have then to
> be the same for both languages to see if there is a
> translation available. I've already figured out that
> I have to do something in the build tab but I can't
> find what exactly I need to change.

Multilingual sites work one of two ways.  
One) they use different urls for different languages e.g.  page?lang=en These
are easy, you can mirror everything or filter on one lang.

Two) A link sets a cookie for the wanted language. For this you create N
different mirrors starting at the link that sets the language and filter out
the other language setting links.