Thomas,
Here is some information about setting up your DotNetNuke site with multiple languages:
After setting up your DotNetNuke site, you'll want to install the Dutch and French language packs that are available as downloads from the DotNetNuke site.
The Admin -> Languages page will allow you to install and enable a language pack.
The Admin -> Site Settings page will allow you to define the default language for a portal. (Advanced Settings -> Other Settings section)
You will also probably want to use a language drop down list in your portal, that will allow the user to select the current language. I believe this can be included in your skin by using the [LANGUAGE] token.
It is important to note that a registered user can pick their own language, which will override the default language for the portal. They can define that in their user account settings. When a user is registered, by default, they will receive the portal's current language. This also means that any users that are registered before you install new languages will be using the English language (which is DotNetNuke's default) -- this includes Host and Admin users.
Setting up the portals with two languages isn't generally a big deal, it's deciding how to display your site's content in multiple languages and customizing third party modules to do so that requires more time and energy. Unfortunately, I do not have any experience with the store front module....
Take care,
Ian