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}]]

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

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

The language icon