drupal multisite htaccess: rewrite all languages to default language on one subsite - regex

Question

I have a Drupal 6 multisite, with 2 domains (www.example.com and www.domain.com), sharing some common content.
The domain example.com is in three languages (EN, FR, NL). Languages are set by path prefix (/en, /fr, /nl). The other domain domain.com is just in one language (NL).
The problem: on many occasions domain.com is shown in the wrong language, even if no path prefix is filled in. Somehow it seems to default to EN, though it doesn't always do that - behaviour doesn't seem to be very consistent.
The solution (at least I hope): since I'm not a Drupal developer (I inhereted the site from a former colleague) I have no idea how to fix this in Drupal, so I thought the best way to fix it would be to add some rewrite rules to .htaccess.
I'm no htaccess/regex expert either, and can't get it working. You can find my current rewrite rules below, any help or suggestions are most welcome.
Some examples:
www.domain.com/fr/some-title needs to be rewritten to www.domain.com/nl/some-title
www.domain.com/node/1975 needs to be rewritten to www.domain.com/nl/node/1975
These are the rewrite rules that were already there:
# Rewrite URLs of the form 'x' to the form 'index.php?q=x'.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !=/favicon.ico
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
I tried adding this:
RewriteCond %{HTTP_HOST} ^(www.)?domain.com$ [NC]
RewriteRule ^/(.*)$ /nl/$1
and would expect this just to prepend /nl/ to all paths (thus not being a full solution since /fr/some-title would become /nl/fr/some-title) - however, a quick test shows me that:
/fr/some-title is rewritten to /nl/some-title (which is what I need, but not what I expected)
/some-title is not rewritten
The question: any ideas what might be wrong? Or could this be caused by other (Drupal) settings? Or is there a better way to solve my problem?
Just for the sake of completeness: the live website is www.cinemazuid.be

Answer 1

If this rule
RewriteRule ^/(.*)$ /nl/$1
is in your .htaccess file, I am surprised that it works as the leading / is always stripped out, so it should theoretically never match any request.
If your desire is to force a default language of NL for those requests that do not specify a language, then add the following rules to the top of your .htaccess file, before any existing rules
#if request is for existing file or directory
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
#then stop processing
RewriteRule .* - [L]
#replace fr with nl. This rule
RewriteRule ^fr/(.*)$ /nl/$1 [L,R=301]
#if the request does not have a language of en or nl
RewriteCond %{REQUEST_URI} !^/(en|nl)/ [NC]
#redirect with nl as default language
RewriteRule .+ /nl%{REQUEST_URI} [L,R=301]
If you do not want to redirect, just drop the R=301
I edited code above to replace /fr/some-title with /nl/some-title/.
The L flag tells mod_rewrite to stop processing further rules, which is usually what you want, unless you have another rule that needs to further process the current request.

Answer 2

#redirect /fr/* and /en/* to /*
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.com$ [NC]
RewriteRule ^(en|fr)/(.*)$ /$2 [R,L]
#internally rewrite /* to /nl/*
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.com$ [NC]
RewriteCond $1 !^nl/$ [NC]
RewriteRule ^(.*)$ /nl/$1
#drupal code
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !=/favicon.ico
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]