Change title: create formal german translation

I mixed up the language-code for formal and non-formal german

translationfiles/de_DE/cookbook.po

@@ -10,7 +10,7 @@ msgstr ""
 "PO-Revision-Date: 2019-08-08 14:38+0200\n"
 "Last-Translator: Christian Wolf <christian.wolf@aut.uni-saarland.de>\n"
 "Language-Team: German <kde-i18n-de@kde.org>\n"
-"Language: de_DE\n"
+"Language: de\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"