It seems there is something wrong with the spell checker.
Although I placed ...
i18n::set_locale('de_DE');
i18n::set_default_lang('de_DE');
i18n::get_tinymce_lang('de_DE');
... into mysite/_config.php the TinyMCE UI is English and the spell checker checks for English grammar. But the little ABC icon shows me after a click "German (Germany)". Anyway it uses the English grammar and makes almost every word red underlined.
How can I change that?