Something weird is happening with language files on my site

As prescribed, I translated the English language strings into Dutch through admin panel.

A hell of a job, but I managed

My site is nearly ready now to publish (except for the database problem) and meanwhile I was testing some things.

I set Dutch as default language but English stays an option, the button (with flag) is on my site.

So I switched language into English using that button, but every time I click on a menu item, my entire site goes back to the 'default' Dutch language.

Someone else encountering this problem ?

You can try it out on www.onderweg.be/index.php (type the complete URL because there's also a index.html in my script to prevent people to see the site)

O yes .... thank you 'visitor' for sending me 2 greetings  

It seems that this problem is still not being solved ... I hate to get greetings from anonymous people :-)