Thanks for your inquiry.
The Font.Bidi property specifies whether the contents of this run shall have right-to-left characteristics; therefore for RTL text (in case Font.Bidi returns true), you can check for the language identifier in Font.LocaleIdBi property.
The Bidi property, when on, shall not be used with strongly left-to-right text. Any behaviour under that condition is unspecified. This property, when off, shall not be used with strong right-to-left text. Any behavior under that condition is unspecified.
When the contents of this run are displayed, all characters shall be treated as complex script characters for formatting purposes. This means that BoldBi, ItalicBi, SizeBi and a corresponding font name will be used when rendering this run.
Hope this answers your query. If you still face problem, please share your document along with code here for investigation purposes.