Hello,
on my “static” sites I use the Website Meta Language 
to create macros. To create the HTML code for a language that is used on a linked website I simply type <flag de> or <flag gb>.
With the WordPress plugin Embedder I can achieve nearly the same. Embedder even supports PHP code. But as my PHP knowledge is limited I only created some shortcuts to mark the language that is used on a linked website. So with WordPress I have to use [flag_de] or [flag_gb] for the moment.
In the past I got the flag images from the World Flag Database . But they only allow the use of up to four images from their database on a website. A source without this limitation is Free World Flags . This site only ask for a link back to their site to use the flag images. That was easy! By the way, I scaled the flag images down to 16×10 for better use in a text like this.
Bye, Tore