/
  /en/
    index.php
    about-us.php
    faq.php
    ...

  /fr/
    index.php
    about-us.php
    faq.php
    ...

  ...
  etc.

# allows for extension-agnostic urls
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*)$ $1.php

RewriteEngine On

# defaults to the english site
RewriteRule !^[a-z]{2}/ /en%{REQUEST_URI} [L,R=301]

# allows for extension-agnostic urls
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*)$ $1.php

Not Found

The requested URL /about-us was not found on this server.

RewriteRule !^(fr|en)/ /en%{REQUEST_URI} [L,R=301]

RewriteRule !^[a-z]{2}/ /en%{REQUEST_URI} [L,R=301]