Multilingual website go live in one language only for now

I am creating a multilingual website.
However, the client is not keeping up with the translations,
so for now only the German version should go live.
What is the best way to do this?
It is important that the language abbreviation is preserved, i.e.

Not quite sure I understand what you want to achieve, or what you mean by

When a page is not translated, Kirby will fall back to the default language version. Unless you want to prevent access to untranslated/partly translated languages, in which case you have to redirect your visitors.

Or not enable the language navigation for the time being.

80% of the pages are already translated. With the remaining 20% it will still take time until they are translated. Therefore, the page should still go online, but only in the German version. If I hide the language switcher in the navigation, the English pages are still accessible because some links from the old page match and are present in Google. Therefore, my question if there is a basic function to unlock only the main language for now.

Well, you can use a route to redirect all English links to the German version for the time being (or to the error page)