This is actually pretty simple since its just two images that need to be linked. In your header.php replace:
<a href="http://www.recital-audio.com/fr"><img src="yourimage.png" /></a>
<a href="http://www.recital-audio.com/en"><img src="yourimage.png" /></a>
Just replace yourimage.png with the url for the image of the flag you want to use for each item. I've added in the links since you already provided them.
Then, in your style.css add the following all the way at the bottom of the files:
It may need a bit of adjusting to your exact taste, but that should get you 90% of the way there :)