rewrite rule check for string existence - regex

Question

i need a RewriteRule which changes an url of http://domain.org/foo/bar to http://domain.org/de/foo/bar but does nothing if there is an url like http://domain.org/en/foo/bar or http://domain.org/de/foo/bar
so if there is no en/ and no de/ then it should add de/, else it should do nothing
how can this be done? i already played around a bit with regex, but I dont know how to check if there is en/ or if there is no en/.
help is highly appreciated.
UPDATE:
my current rules are the default rules you should have when running wordpress:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
UPDATE2:
this is wordpress and de/ and en/ are not actual folders.. they are used by a multilanguage plugin. since i have some issues with this plugin i need the redirect as described above.

Answer 1

In general without any other rules, this would redirect anything that doesn't start with /en/foo/bar.
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !^/en [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /de/$1 [R=301,L]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

Answer 2

You can use this rule in your root .htaccess:
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^((?!(?:en|de)).*)$ de/$1 [L,NC,R]
Keep this rule before default WP rule.
EDIT Full .htaccess:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^((?!(?:en|de)).*)$ de/$1 [L,NC,R]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress