I noticed that there are still some missing translations in the stable version of Zotero 6. I tried requesting to join the German translation team on Transifex, but it doesn't let me select "German" since that language already exists. So I'll just quickly make some suggestions for some of the most obvious missing strings.

Add to Collection - Zur Sammlung hinzufügen
Annotations - Anmerkungen (that's how Apple translates "annotations" e.g. on https://support.apple.com/de-de/guide/preview/prvw11580/mac, although it could also be "Annotationen" or "Markierungen", which is what Citavi calls it)
Add Note from Annotations - Notiz aus Anmerkungen hinzufügen (instead of "hinzufügen" it could also be "erstellen", which would be a bit shorter, but literally means "create")
Export Note... - Notiz exportieren...

In PDF Reader:
Show in Library - In Bibliothek anzeigen
Move Tab - Tab verschieben ("verschieben" is what Firefox uses, Safari uses "bewegen")
Move to Start - An den Anfang verschieben
Move to End - An das Ende verschieben
Move to New Window - In neues Fenster verschieben
Close - Schließen
Close Other Tabs - Andere Tabs schließen
Reopen Closed Tab - Geschlossenen Tab erneut öffnen ("erneut öffnen" literally means "reopen" and is used by Safari; Firefox uses "wiederherstellen", which means "restore")
Search Annotations - Anmerkungen durchsuchen
Edit Page Number... - Seitenzahl bearbeiten...
Edit Highlighted Text... - Markierten Text bearbeiten...
Add an annotation to see it in the sidebar - Fügen Sie eine Anmerkung hinzu, um sie in der Sidebar zu sehen ("Anmerkung hinzufügen, um sie in der Sidebar zu sehen" would probably also work, but the former is more polite)