<Style x:Key="Custom.ToggleSwitch.Win10"
       BasedOn="{StaticResource MahApps.Metro.Styles.ToggleSwitch.Win10}"
       TargetType="{x:Type Controls:ToggleSwitch}">
    <Setter Property="Padding" Value="0 0 10 0" />
    <Style.Triggers>
        <Trigger Property="ContentDirection" Value="RightToLeft">
            <Setter Property="Padding" Value="10 0 0 0" />
        </Trigger>
    </Style.Triggers>
</Style>

BasedOn="{StaticResource MahApps.Metro.Styles.ToggleSwitch.Win10}"

<Window.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.ToggleSwitch.xaml" />
        </ResourceDictionary.MergedDictionaries>
        <Style x:Key="Custom.ToggleSwitch.Win10"
               BasedOn="{StaticResource MahApps.Metro.Styles.ToggleSwitch.Win10}"
                TargetType="{x:Type Controls:ToggleSwitch}">
            <Setter Property="Padding" Value="0 0 10 0" />
            <Style.Triggers>
                <Trigger Property="ContentDirection" Value="RightToLeft">
                    <Setter Property="Padding" Value="10 0 0 0" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </ResourceDictionary>
</Window.Resources>

<Controls:ToggleSwitch Style="{StaticResource MahApps.Metro.Styles.ToggleSwitch.Win10}" />