The locale used to select region-specific glyphs.
This property is rarely set. Typically the locale used to select
region-specific glyphs is defined by the text widget's BuildContext
Localizations.localeOf(context). For example RichText defines
its locale this way. However, a rich text widget's TextSpans could
specify text styles with different explicit locales in order to select
different region-specific glyphs for each text span.
final Locale locale