This is an optional language tag for all types of material in X5 and X6 apart from music videos. Its definition is "A code to identify the language of closed captions - optional since IMSC1.1 subtitles carry the language tag in their own document. Useful for OP-47 subtitles if supported." This definition implies strongly that it shouldn't be there unless OP-47 subtitles are present (and which are not supported by X5), but it might be worth checking during file validation.
This is an optional language tag for all types of material in X5 and X6 apart from music videos. Its definition is "A code to identify the language of closed captions - optional since IMSC1.1 subtitles carry the language tag in their own document. Useful for OP-47 subtitles if supported." This definition implies strongly that it shouldn't be there unless OP-47 subtitles are present (and which are not supported by X5), but it might be worth checking during file validation.