The icons are located in images:flag_icons.

Example of code that shows the japanese flag if the corresponding page exist. First code that's based on the name of the current page. 

(:if exist WebJa.{*$Name} :)
[[images:lang_icons/Ja.png -> WebJa.{*$Name}]]
(:ifend:)
?

Then code where we fake the name of the page, for testing purposes. 

(:if exist WebJa.Home :)
[[images:lang_icons/Ja.png -> WebJa.Home]]
(:ifend:)

The language icon

From http://www.languageicon.org/

FileIcon
...r9_c24
...r9_c23
...r9_c22
...r9_c21
...r9_c20
...r9_c19