I am attempting to redirect articles from the blog for their british version.

Essentially, articles /my/article is within french. But you have access to the british version at /en/my/article. I have to redirect for this url when the user isn't french.

I chose to make this rule during my .htaccess :

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP:Accept-Language} !^fr [NC]
RewriteCond %{REQUEST_URI} !^/en [NC]
RewriteRule ^(.*)$ /en/$1 [L,R]
</IfModule>

When I am likely to /my/articles I am really rerouted to /en/index.php. I believe it's due to the Wordpress default redirection:

<IfModule mod_rewrite.c>
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

How do i get this to work?

Update the wordpress_options table inside your WordPress database. Look for "/my/" in wordpress_options and replace all cases of it with "/en/".