Hi Chouby,
Four months passed since this discussion. I'm starting a new site this time using suffusion theme. This site will also be multilingue and I'm facing again this same issue with wp-seo.
When you made the test, did you change wp-config.php at language definition to define('lang', 'pt_PT') ?
Suddenly occurred to me that in a multilingue site this definition doesn't make sense and might conflict with your plugin's base language function.