Frederic Wenzel has an interesting article about inserting "language" icons through CSS. Two things have to be noted, one is historical but the other is rather important :

  1. this is oooooooooooooold story...
  2. a country flag is NOT relevant when dealing with languages. What's the flag for Kurdish ? Or for chinese dialects or distinct languages in China ? What are the two flags for Nynorsk and Bokmål in Norway ? What's the flag for French in African countries where French is an/the official language ? What's the flag for Aramaic (yes, the language Jesus spoke if he existed, and I remind you this is a live language...) ? What's the flag for Esperanto ? What's the flag for the sicilian dialect of Italian ?

Flags don't carry enough information. Flags can be extremely controversial since they unfortunately give status of "nation" to the speakers of a given language. I do not recommend using flags for languages on for instance...

In fact the best way to tag the language of a link is the following one:

a[hreflang|="fr"]:after {
  content: " [target's language is French]";
  font-style: italic;

or the generic

a[hreflang]:after {
  content: " [" attr(hreflang) "]";