It is enabled by default. One creator of English language content had their video misclassified as Spanish and people were getting a machine English dub on an English video. Support to fix it appears to be a nightmare.

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the_af 3 days ago | parent [-]

Wait, do you mean it's enabled by default but the author can disable it?

If not, I wonder why I can still watch most videos in their original language (even though I'm in a Spanish-speaking country), and I only encountered this once so far.

	▲	extraduder_ire 3 days ago \| parent \| next [-]
		It's being gradually rolled out to channels. Will likely hit larger ones earlier, or following some other metric youtube is following.
	▲	kevin_thibedeau 3 days ago \| parent \| prev [-]
		There is supposed to be a procedure to manually remove each language dub but it was broken (at least as of last year).