In WordPress (4.x) English version, Site Language option is missing from the admin Settings > General window.
You can restore Site Language by downloading language packages and place them in the specific directory.
First, go to the locale page of your language from here and download language package file.
Next, unzip the file, open wp-content and copy languages to the /var/www/html/wp-content of your WordPress.
After copying languages, go back to the Settings > General and you should be able to see the Site Language option.