WordPress site redirects

If I understand correctly you mean to check the permalinks in the page’s menu? It’s correctly. So in the WP-admin it tells me the link is fosst.nl/wp/menuitem, but when i click it on the website it goes to fosst.nl/nl/wp/menuitem

They are selected as pages.

Yes! Apologies for my late response, i was gone for a few days:

The HTACCESS of the main directory, our Joomla website:

##
# @version $Id: htaccess.txt 10492 2008-07-02 06:38:28Z ircmaxell $
# @package Joomla
# @copyright Copyright (C) 2005 - 2008 Open Source Matters. All rights reserved.
# @license http://www.gnu.org/copyleft/gpl.html GNU/GPL
# Joomla! is Free Software
##

#####################################################
#  READ THIS COMPLETELY IF YOU CHOOSE TO USE THIS FILE
#
# The line just below this section: 'Options +FollowSymLinks' may cause problems
# with some server configurations.  It is required for use of mod_rewrite, but may already
# be set by your server administrator in a way that dissallows changing it in
# your .htaccess file.  If using it causes your server to error out, comment it out (add # to
# beginning of line), reload your site in your browser and test your sef url's.  If they work,
# it has been set by your server administrator and you do not need it set here.
#
#####################################################

#php_flag display_errors on
#php_value error_reporting 32767

Options +FollowSymLinks

RewriteEngine On

########## Begin - Rewrite rules to block out some common exploits
## If you experience problems on your site block out the operations listed below
## This attempts to block the most common type of exploit <code>attempts</code> to Joomla!
#
# Block out any script trying to set a mosConfig value through the URL
RewriteCond %{QUERY_STRING} mosConfig_[a-zA-Z_]{1,21}(=|\%3D) [OR]
# Block out any script trying to base64_encode crap to send via URL
RewriteCond %{QUERY_STRING} base64_encode.*\(.*\) [OR]
# Block out any script that includes a <script> tag in URL
RewriteCond %{QUERY_STRING} (\<|%3C).*script.*(\>|%3E) [NC,OR]
# Block out any script trying to set a PHP GLOBALS variable via URL
RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]
# Block out any script trying to modify a _REQUEST variable via URL
RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2})
# Send all blocked request to homepage with 403 Forbidden error!
RewriteRule ^(.*)$ index.php [F,L]
#
########## End - Rewrite rules to block out some common exploits

#  Uncomment following line if your webserver's URL
#  is not directly related to physical file paths.
#  Update Your Joomla! Directory (just / for root)

#RewriteBase /pendragon/

########## Begin - Joomla! core SEF Section
#
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
#
# If the requested path and file is not /index.php and the request
# has not already been internally rewritten to the index.php script
RewriteCond %{REQUEST_URI} !^/index\.php
# and the request is for root, or for an extensionless URL, or the
# requested URL ends with one of the listed extensions
RewriteCond %{REQUEST_URI} (/[^.]*|\.(php|html?|feed|pdf|raw))$ [NC]
# and the requested path and file doesn't directly match a physical file
RewriteCond %{REQUEST_FILENAME} !-f
# and the requested path and file doesn't directly match a physical folder
RewriteCond %{REQUEST_FILENAME} !-d
# internally rewrite the request to the index.php script
RewriteRule .* index.php [L]
#
########## End - Joomla! core SEF Section

The one of the WP site:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress