Hi Mats,
You can change the default language by using a custom configuration file for CKeditor as described here:
http://www.mojoportal.com/customizing-the-editor-toolbars.aspx
in your custom file you can set the default like this:
config.scayt_sLang = 'de_DE';
ref CKeditor documentation
http://docs.cksource.com/ckeditor_api/symbols/CKEDITOR.config.html#.scayt_sLang
I do not think it should adapt to the browser language it should be based on the site language.
Hope it helps,
Joe