Hi @chrischo68,
There is no general option for users to change the language of Planner notifications, but there is an options to change the langauge of notifications for the plan (group for notifications).
You can change this in Planner by choosing the plan for which you want to change the language settings, then by clicking on the "plan settings" and then on the "group settings" (it is important to know that only group owner or admin can see and change these settings):
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Kindest regards,
Leon Pavesic