Hi Daniel,
Thanks for using Syncfusion products.
In order to apply the localization for editor form, you can use the AppointmentForm.DisplayStrings for appointment form. Please refer the below code snippet and refer the below KB link,
Code snippet
//Localization for scheduleGrid
this.scheduleControl1.Culture = new CultureInfo("es-ES");
//Localization for Enter Appointment in Appointform.
AppointmentForm.DisplayStrings[11] = "Introducir cita";
Please refer the below KB link:
https://www.syncfusion.com/kb/7104/how-to-localize-the-appointment-form-default-strings-in-schedulecontrol
Regards,
Mohanraj G.