All theme translations are provided to us by members of the community, so they are updated only as they are sent to us - you don't pay separately for these language files. The translation files often only need be updated for major releases when the most changes are made (e.g. v1.1 or v1.2..), however sometimes you may need to update for minor releases as well (e.g. v1.1.2, v1.1.4..). We would recommend they're reviewed at every release anyway so you're not left with untranslated strings.
As per the link provided, you can create your own translation file if one does not exist for the latest version, which would give you the translation you require. We do offer translator benefits as well, so you could save money on your theme purchase if you were to provide an update German translation for the theme. More information is on the language page itself. Thanks.