Site Language Definition solves a common problem: your website is in your chosen language but for some reason the browser 'thinks' it's in another language.
It is particularly common to think that your site is in a different language — many Chrome users get messages from Chrome along the lines of, "This site is in Indonesian. Would you like Chrome to translate it?" even when the site doesn't have a hint of Indonesian! This has very negative SEO ramifications: Google prioritizes in its search results sites that it knows are in the user's language.
Site Language Definition plugins adds the necessary language attributes to your website to force the browsers see the website as configured under the WordPress General Settings.
If you have any other suggestions, please let us know! You can contact us via http://www.wpsos.io/wordpress-plugin-site-language-definition/
For more information and support, check out: http://www.wpsos.io/wordpress-plugin-site-language-definition/