Mb 02 Update the defaultLanguageFlag chapter.

Add a tip and a screenshot as well. I don't know since when exactly the location is now :file:`typo3/sysext/t3skin/images/flags`. But I don't think is important to waste time on that.
Please use a figure directive instead. Furthermore we place all images in "Documentation/Images" and not along the reST files. Also we use UpperCamelCase for file names.