rewrite working on localhost but not on server - php

Question

I'm uploading my website on my server. The rewrite rules above were working on my localhost. URL "mywebsite.com/about" served me "mywebsite.com/about.php". But something weird happens.
My htaccess file has got the right autorizations, I Know that because the other rules (deflate for exemple) are working correctly.
mod_rewrite is on, I know that because some of my rules are working. However, not everything : when typing "mywebsite.com/about" or "mywebsite.com/about.php", I get redirected to "mywebsite.com/about", but it displays a 404 error.
ErrorDocument 404 /includes/404.php
Options +FollowSymlinks
Options +MultiViews
DefaultType application/x-httpd-php
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^$ /en [L]
### to hide '.php' extensions
# Unless directory, remove trailing slash
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/$ mywebsite.com/$1 [R=301,L]
# Redirect external .php requests to extensionless url
RewriteCond %{THE_REQUEST} ^(.+)\.php([#?][^\ ]*)?\ HTTP/
RewriteRule ^(.+)\.php$ mywebsite.com/$1 [R=301,L]
# Resolve .php file for extensionless php urls
RewriteRule ^([^/.]+)$ $1.php [L]
###
RewriteBase /
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]
</IfModule>
I would really apreciate help on finding where I go wrong!
EDIT:
The page I'm trying to access is actually "mywebsite.com/en/about.php" by the URL "mywebsite.com/en/about"
EDIT 2:
I left nothing but this line on my .htaccess :
# Resolve .php file for extensionless php urls
RewriteRule ^([^/.]+)$ $1.php [NC,L]
What it does is : strictly nothing... If I understand it well, this is the part that should redirect URL with no extensions to the .php version of it. When I go to mywebsite.com/en/about , i get a 404.
EDIT 3:
Using this other script gives the same result : rewriting the URL in "mywebsite.com/en/about" is working, but it doesn't redirect to the .php version.
# Turn mod_rewrite on
RewriteEngine On
RewriteBase /
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.php [NC]
RewriteRule ^ %1 [R,L]
# Resolve .php file for extensionless php urls
RewriteRule ^([^/.]+)$ $1.php [NC,L]