2.1.4 [W3C-TTML-1.0] Section 7.2.2, xml:lang


The specification states:

 The xml:lang attribute must be specified on the tt element and may be specified by an instance of any other element type in the core vocabulary catalog.

IE10 Mode, IE11 Mode, and EdgeHTML Mode (All Versions)

The xml:lang attribute is ignored when selecting a track to display.