To successfully setup a non english site you have to adjust several settings in typoscript.
Here is a list of them with explanation:
Define wich localization mark will be used by typo3. Especially the predefined translations of extensions will use this setting. But you also can use this in your own extensions and typoscript.
Set the PHP-Locale. This setting will be used by all localized php function like stftime. Especially the displaying of dates is customized this way
Setting the language key for the html/xml document means being nice to search engines because they dont have to guess the language of your site. So just do it.
If you found a mistake or if you have a good tip or suggestion, please contact me at thomas(at)typo3-unleashed.net. I will check it and correct the entry or insert the new one.
Thanks in advance for your help.