When choosing Belgium as regional settings, the installation switch to dutch 
language. Perhaps there should be to locales : Belgium-Dutch and 
Belgian-French or a way to keep english language.

There will be a button 'reset to English' for the language.
That way you still have settings for Belgium
Do the two parts of Belgium use different settings (language ofcourse, but what
else? keyboard layout?)
I'll see what I can do to improve things a bit.