WordPress uses the gettext function to manage i18n and l11n, so adding or changing a string is a little bit more complicated than just simply opening up a source file and editing the string.
Specifically, the language strings are contained in the language files located in the theme’s languages folder. For the German language, this file is called de_DE.po. If this language file is not updated since the last theme update, the new language strings will not be there. So first you need to update the language file to include the new strings, then you need to translate those strings, then you need to convert that .po file in a machine object, a .mo file.
I find it easiest to use softwares like POEdit or the Codystyling Localization plugin for wordpress to do this.