You can learn more about how this works on the dedicated YouTube Support pages for the comment translate feature. Over 100 languages are available including Spanish, Portuguese, German, French, Russian, Mandarin, English (obviously), and many, many more. The option is part of the regular YouTube Experiments that offers Premium subscribers the opportunity to try out features likely ahead of a wider rollout on the platform. You do need to opt in to actually access YouTube comment translation or the “Translate” toggle within the video comment section, and restarting the app should be enough to get the feature to start working correctly (h/t Android Police).
If you have a Premium membership plan, YouTube for Android and iOS is now testing quick comment translations. Following a trial period with YouTube Premium subscribers, comment translations are now available to everyone on mobile — iOS and Android — as announced by the official @TeamYouTube Twitter account: To get started, follow the same steps as during the initial preview period for Premium subscribers. The “translate” option under most comments converts the text to your local language, which is recognised depending on your location and previously watched movies. This should imply that if you view a lot of videos in a given language, the material will convert to that language.
Now, I can’t honestly say with certainty just how good these YouTube comment translations are, but they seem a little off the mark at times — especially given video context. That said, we all know just how much of a minefield YouTube comment sections can be without a translate feature, so maybe we’ve been spared some typically terrible responses for years. You can give feedback using the dedicated questionnaire if you think the feature isn’t as good as it should be. If you want to try YouTube comment translations for yourself and you’re already a Premium subscriber, then you can sign up here. It’s also worth noting that it’s only available until September 9. After that date, it’s not clear if we’ll see it come to roll out more widely, but it’s probably about time the option came to YouTube.
When translation is live on your iOS or Android device, as long as YouTube detects that a comment is in a different language to your preset device/app settings, then a “Translate” toggle with the Google Translate icon will appear underneath the foreign text. Tapping converts it to your local language, and switching back is as easy as tapping “See original”: Tap the ‘Translate’ button on comments written in languages that you don’t understand. You can always switch back to the original language by tapping ‘See original’.