On small width, the Headertext is not complete visible. Please have a look at attached picture.
Comments: ** Comment from web user: shawnoster **
The issue is that the Header is represented by a ContentControl in the template which provides the maximum benifit for re-templating but you no longer have the benefit of setting properties such as word wrap. One solution would be to change Header from a ContentControl to a TextBlock but this might break apps expecting to be able to put whatever they'd like in that space.
The current solution is to pass in your own Header template that has word wrapping turned on, like this:
<toolkit:ToggleSwitch Header="This is an example of a really long description label for localization">
<toolkit:ToggleSwitch.HeaderTemplate>
<DataTemplate>
<TextBlock FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
Foreground="{StaticResource PhoneSubtleBrush}"
TextWrapping="Wrap"
Text="{Binding}" />
</DataTemplate>
</toolkit:ToggleSwitch.HeaderTemplate>
</toolkit:ToggleSwitch>