In Liferay you can setup (portal-)languages which are available to sites.
In control panel for a site, you can choose which of these languages should be available to a user of that site.
I want to change the flag of the default language porlet in liferay 6.2.
According to this forum, it looks like the only way to change those images is to replace the files manually.
You can find them under
portal-web/docroot/html/themes/_unstyled/images/language
and the icons should be the size of 16x11px.