I'm going to take a wild guess here, so could be completely useless...
RewriteCond $1 !\.(gif|jpe?g|png)$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /index.php/$1 [L]
RewriteRule /international/(.*)$ /index.php/$1 [L]
The problem looks like you rewrite the URL adding in the index.php?, then add it in again for the international - hence when you add "?" it to both, it kills the rewrite.
So for the first rewrite you need to exclude international, and for the second ensure it's only effecting international.
RewriteCond $1 !\.(gif|jpe?g|png)$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(international)
RewriteRule ^(.*)$ /index.php?/$1 [L]
RewriteCond %{REQUEST_URI} (international)
RewriteRule /international/(.*)$ /index.php?/$1 [L]
You may need to repeat the 3 initial conditions for the second part.
UPDATE
This got it working in the end (Thanks for pointing me in the right direction - Luke)
RewriteCond $1 !\.(gif|jpe?g|png)$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(international)
RewriteRule ^(.*)$ /index.php?/$1 [L]
RewriteCond $1 !\.(gif|jpe?g|png)$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} (international)
RewriteRule ^(.*)$ /index.php/$1 [L]
RewriteRule /international/(.*)$ /index.php?/$1 [L]