When printing the name of the video track on the OSC, mpv almost always
prefixes it with "unknown". This is referring to the language tag which
essentially nobody ever does for the video track since it doesn't make
any sense. Instead of putting unknown when there's no language, just
make it an empty string instead. "Unknown" confuses can confuse users
here and practically nobody would guess that it's referring to language.
We still will print the language for a video track in case someone out
there really does tag it, but that should be rare.
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