This article contains more info about multilingual setup (especially the language switcher section at the end). http://ez.no/community/articles/new_features_in_ez_publish_3_8/translating_and_displaying_multi_lingual_content
The problem with the database error is most likely that the site.ini of your siteaccesses en and nl do not contain any database settings. Check the site.ini of the siteaccess that was created during installation, there should be a block [databasesettings] there. Either copy this block into the site.ini of each siteaccess or put it in the site.ini of the override folder.
If you still have problems, post again, but then also please post the site.ini of e.g your 'en' siteaccess (don't forget to remove any passwords).
Hmm, check whether you have a setting RequireUserLogin in the block [siteaccesssettings] of the site.ini of the 'nl' access. Make sure you have cleared all the cache and then try again.
Theoretically the [rolesettings] in the site.ini could also cause this problem, but these settings should be fine unless you changed them (usually this block is only in the general site.ini in the settings folder, don't change anything there).
BTW: If you EZ 3.9 I can't help you - I haven't looked at it yet so I don't know what settings there are in this version.
RequireUserLogin was there, but set to false. Removing it, removes the Denied Access error, but now the hompage does not show the folder description which is showing in the english version. Does that mean my translation of that folder is incorrect?
Board member, eZ Publish Community Project Board - Member of the share.ez.no team - Key values: Openness and Innovation.