Hi Antonio,
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 attached sample,
Code snippet
//Localization for scheduleGrid
this.scheduleControl1.Culture = new CultureInfo("es-ES");
ScheduleGrid.DisplayStrings[4] = "Doble clic para añadir todos los días del evento";
//Localization for Appointform.
AppointmentForm.DisplayStrings[0] = "Tema";
AppointmentForm.DisplayStrings[1] = "Ubicación";
AppointmentForm.DisplayStrings[2] = "Mostrar el tiempo";
AppointmentForm.DisplayStrings[3] = "Recordatorio";
AppointmentForm.DisplayStrings[4] = "Evento";
AppointmentForm.DisplayStrings[5] = "Hora de inicio";
AppointmentForm.DisplayStrings[6] = "Hora de finalización";
AppointmentForm.DisplayStrings[7] = "Etiqueta";
Please refer the below KB link,
Regards,
Mohanraj G.