tonihei e5aed73fba Switch format language tag to use full BCP 47 codes.
This allows to distinguish between regional variants and scripts.

We still need to normalize the language code itself to make track selection
independent of the whether 2 or 3 letter codes are used.

PiperOrigin-RevId: 239783115
2019-03-22 19:56:25 +00:00
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