htaccess ignore specific directory in URI - treat same as root - regex

Question

been looking for several hours now without success.
I'd like all urls containing /en/ to ignore this part but continue with all other rules in the htaccess (like php to no extension rule).
EXAMPLE
www.domain.com/en/gallery -> should point to gallery.php in my server root
www.domain.com/gallery -> should ALSO point to gallery.php in my server root
(will have to take care of duplicate content here somehow later)
www.domain.com/en/contact -> should point to contact.php in my root
Thank you
U P D A T E
Like mentioned in my question I'd like all other rules to still be running.
Only this rule does not work with below solution as it should work with
www.domain.com/en/category1 AND www.domain.com/category1 so with or without EN.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !(\.png|\.jpg|\.gif|\.jpeg|\.bmp|\.css|\.js|\.ico)$
RewriteRule ^([^/]+)/? /categories.php?rw=1&url=$1 [L,QSA]

Answer 1

Put this code in your DOCUMENT_ROOT/.htaccess file:
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}/$1.php -f
RewriteRule ^(?:en/)?([^/]+)/?$ $1.php [L]
UPDATE:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !\.(png|jpe?g|gif|bmp|css|js|ico)$
RewriteRule ^(?:en/)?([^/]+)/?$ categories.php?rw=1&url=$1 [L,QSA]