This is controlled by google. There is nothing I can do to change it's behavior or customize it to use flags.
I don't like this feature myself because if a visitor comes to your site with their browser preferred language set as something other than the same language the site uses then it forces a big toolbar at the top that pushes down the design and throws off the layout. When I first implemented this in mojoPortal I did not realize it was doing this for all foreign visitors, I later removed it from my own site because of this.
I actually also implemented a control for Microsoft Translate because I "thought" it did not force a toolbar at the top, but it turns out it does the same thing if the browser language setting is not the same as the site content.
hopefully one day one of these vendors will give us more control over the translate widgets.
Best,
Joe