Fixed: Spellcheck in Magento Admin

This is a quick solution to get spellcheck in magento working.
It worked for me with magento 1.6.2.
The best solution is the creation of an extension, but I didn't take the time.
Also beware that with the next magento update the changes will be overwritten.

 1) Add the red parts to \js\mage\adminhtml\wysiwyg\tiny_mce\setup.js

        theme_advanced_buttons1 : magentoPlugins + 'magentowidget,bold,italic,underline,strikethrough,|,justifyleft,justifycenter,justifyright,justifyfull,|,styleselect,formatselect,fontselect,fontsizeselect',
            theme_advanced_buttons2 : 'cut,copy,paste,pastetext,pasteword,|,search,replace,|,bullist,numlist,|,outdent,indent,blockquote,|,undo,redo,|,link,unlink,anchor,image,cleanup,help,code,|,forecolor,backcolor',
            theme_advanced_buttons3 : 'tablecontrols,|,hr,removeformat,visualaid,|,sub,sup,|,charmap,iespell,media,advhr,|,ltr,rtl,|,fullscreen,|,spellchecker',
            theme_advanced_buttons4 : 'insertlayer,moveforward,movebackward,absolute,|,styleprops,|,cite,abbr,acronym,del,ins,attribs,|,visualchars,nonbreaking,pagebreak',
            theme_advanced_toolbar_location : 'top',
            theme_advanced_toolbar_align : 'left',
            theme_advanced_statusbar_location : 'bottom',
            theme_advanced_resizing : true,
            convert_urls : false,
            relative_urls : false,
            content_css: this.config.content_css,
            custom_popup_css: this.config.popup_css,
            magentowidget_url: this.config.widget_window_url,
            magentoPluginsOptions: magentoPluginsOptions,
            spellchecker_languages : 'English=en,+Deutsch=de',
            ......

Add the spellchecker_languages (see lines above) according to you needs:
This enables you to specify what languages your pspell installation can handle. The value of this option should be a comma separated name value list in the following format name1=value1,name2=value,name3=value where name is the string to present in the menu and the value is a ISO language code like sv or en. If you add a + character infront of the name it will be the default value for the spellchecker. The default value for this option is: +English=en.

 2) Go to http://www.tinymce.com/download/download.php  and get the PHP Spellchecker.

3) Put the folder spellchecker from this zip inside \js\tiny_mce\plugins

That's all. Have fun.