With LIB version 2015.2.907 we changed the localization implementation of BusyContent property. Now if you need to show the default text and support changing the culture at run time, you would need to clear the value of BusyContent property and reapply the control template of RadBusyIndicator right after changing the culture:

Thread.CurrentThread.CurrentCulture = new CultureInfo("es");
Thread.CurrentThread.CurrentUICulture = new CultureInfo("es");
this.busyIndicator.ClearValue(RadBusyIndicator.BusyContentProperty);
this.busyIndicator.OnApplyTemplate();

Regards,
Kalin

Available in LIB version 2015.2.720, it will be also available in the 2015 Q2 SP1.