4.8.9 The track element http://www.whatwg.org/specs/web-apps/current-work/#the-track-element ... The srclang attribute gives the language of the text track data. The value must be a valid BCP 47 language tag. This attribute must be present if the element's kindattribute is in the subtitles state. [BCP47] http://www.whatwg.org/specs/web-apps/current-work/#refsBCP47
This is an author requirement, not something that WebKit needs to know about. Please see <http://www.whatwg.org/specs/web-apps/current-work/#how-to-read-this-specification> for more detail.