From https://progress.opensuse.org/issues/46685#note-3

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As of today (beta1), the sidebar is translated (German). But only if you have
clicked "Weiter" for the first time.

Longer version: When you select "German", buttons and other widgets are
displayed in German immediately. But the sidebar will be in English. Then you
click "Weiter" and the sidebar will be in German as well.

I do not check whether everything in Weblate and the packages is fine. But
because the strings are displayed in German, I'd guess that. (In the past,
these sidebar strings were part of desktop-translation. Maybe, this is
different now.)
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("Weiter" is "next")

My guess is that the "on_change" hook of the language chooser updates the
buttons etc., but doesn't update the sidebar.