Can I contribute to the translation of the plugin?
The short answer; yes, you can.
The longer version:
Yoast SEO free
You can help translate our free Yoast SEO plugin by going to WordPress.org. There you can see how many strings are already translated in your language. You can click your language to start contributing. You can also use the WordPress.org translation handbook as a guideline.
To help us translate our Premium plugins in your language, please sign up at http://yoa.st/reg. Once you’re logged in, you’ll be able to add your translations. Once they’re approved, we will include them in the plugin.