how to set language

how can I set the language (en, da, de etc) with mod_rewrite?

all files are located in the same dir

url to set language is

?set_lang=da

rewrite:

www.domain.com/en/index.php                     => www.domain.com/index.php?set_lang=en
www.domain.com/en/another_page.php?cat=black    => www.domain.com/another_page.php?set_lang=en&cat=black

www.domain.com/da/index.php                     => www.domain.com/index.php?set_lang=da
www.domain.com/da/another_page.php?cat=black    => www.domain.com/another_page.php?set_lang=da&cat=black

EDIT:

my .htaccess looks like this

RewriteCond %{HTTP_HOST}        ^domain\.net$ [NC]
RewriteRule ^(.*)$              http://www.domain.net/$1 [L,R=301]

RewriteCond %{HTTP_HOST}        ^www\.domain\.net$ [NC]
RewriteCond %{REQUEST_URI}      !^/_domain\.net/ [NC]
RewriteRule ^(.*)$              /_domain.net/$1 [L]

RewriteCond %{HTTP_HOST}        ^(demo|mysql|secure)\.domain\.net$ [NC]
RewriteCond %{REQUEST_URI}      !^/_domain\.net/_(demo|mysql|secure)/ [NC]
RewriteRule ^(.*)$              /_domain.net/_%1/$1 [L]

RewriteCond %{HTTP_HOST}        domain\.net$ [NC]
RewriteCo开发者_如何学Gond %{REQUEST_URI}      !^/_domain\.net/ [NC]
RewriteRule ^(.*)$              http://www.domain.net/$1 [L,R=301]

I found a solution

RewriteCond %{HTTP_HOST}        domain\.net$ [NC]
RewriteCond %{REQUEST_URI}      ^/(da|en)/(.*)(\?%{QUERY_STRING})?$ [NC]
RewriteRule ^(.*)$              /%2?%{QUERY_STRING}&set_lang=%1 [L]