subtitleLanguage

This term is proposed for full integration into Schema.org, pending implementation feedback and adoption from applications and websites.

Thing > Property > subtitleLanguage

Languages in which subtitles/captions are available, in IETF BCP 47 standard format.
Values expected to be one of these types
Language
Text
Used on these types
BroadcastEvent
Movie
ScreeningEvent
TVEpisode

Source

https://github.com/schemaorg/schemaorg/issues/2110