This is not easy, I’m not 100% .htaccess pro, but I think, this could be a good start:
In your joomla .htaccess, you’ve got something like:
## 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 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.
add before something like:
RewriteRule ^de/theme-t-shirt-shops/apres-ski-t-shirts/(.*)$ de/theme-t-shirt-shops/apres-ski-t-shirts/index.html$1 [L]
But this is untested and I don’t know it if will work in general. Maybe someone else has an idea