When setting the CurrencyDecimalSeparator to "," the MaskedCurrencyInput shows ".," (decimal point is not expected).
Similar issues with MaskedNumericInput.
Setting both CurrencyDecimalSeparator and CurrencyGroupSeparator to "." also produces issues.

Possible workaround:
 public class CustomCurrency: RadMaskedCurrencyInput
    {
        protected override void SetSeparators(params Telerik.Windows.Controls.MaskedInput.Separator[] separatorsArray)
        {
            base.SetSeparators(separatorsArray);
            this.Separators.Add(new Separator(this.Culture.NumberFormat.NumberDecimalSeparator, true));
        }
    }

The fix will be available in lib for 11 July 2016.