The captions in RadScheduler are dynamically set. Therefore, you cannot get your custom strings displayed by changing properties in the Edit UI Elements dialog.
In order to customize the desired strings, you should create your own localization provider. For additional information, please refer to the following article: Translating Strings
. The RadSchedulerStringIds that you need are RadSchedulerStringId.NextAppointment and RadSchedulerStringId.PreviousAppointment.
If you have additional questions, feel free to ask.
All the best,
the Telerik team
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