World Flag is a free WordPress plugin which enables your blog authors to use country flags easily inside their posts, pages and other shortcode supported areas. So if you are looking for such kind of plugin then simply install and activate it.
After activation the plugin automatically adds a Tinymce Editor button present in your add new and edit posts pages. This button makes flag usage very easy for your site authors, all they need to do is click the button then select their flag and insert them inside their content.
Or you can also insert the flags manually by using the shortcode [flag country="COUNTRY_CODE"] Remember to replace COUNTRY_CODE with two letters Standard Country Code. View more screenshots here.