With the help of Mark Hamstra, I finally solved this issue with the following settings:
.htaccess
# For full documentation and other suggested options, please see
# http://svn.modxcms.com/docs/display/MODx096/Friendly+URL+Solutions
# including for unexpected logouts in multi-server/cloud environments
# and especially for the first three commented out rules
# Friendly URLs
RewriteEngine On
RewriteBase /~infodm/ (this was first [b]/%7Einfodm/[/b])
# Fix Apache internal dummy connections from breaking [(site_url)] cache
RewriteCond %{HTTP_USER_AGENT} ^.*internal\ dummy\ connection.*$ [NC]
RewriteRule .* - [F,L]
# Exclude /assets and /manager directories from rewrite rules
RewriteRule ^(manager|assets) - [L]
# Redirect from mydomain.com/rootname to mydomain.com/rootname/
RewriteRule ^en$ en/ [R=301,L]
RewriteRule ^nl$ nl/ [R=301,L]
# The Friendly URLs part
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^en/(.*)$ index.php?q=$1 [L,QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^nl/(.*)$ index.php?q=$1 [L,QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
For otherFlagItemTpl (one of the tpls for language switch) we used:
<li class="yams_lang_(yams_id)"><a href="(yams_docr)?yams_lang=(yams_tag)" lang="(yams_tag)">(yams_name)</a></li>
That extra parameter seemed necessary in IE here.
Check the other settings in the attachment!
