在 quora 上，找来一个办法，我试了下，有效：

Many people complain that the YouTube comment translation function only works intermittently, inconsistently, or not at all. I get it, because it happened to me countless times!

Okay, here’s a solution that actually works, but warning, it may take a couple days to begin working (as apparently YouTube takes time to process your changes). But it will eventually work, and work permanently (i.e., it has been working for me and others for months, and hopefully forever).

Here we go:

Open the YouTube app on your phone. Click on your pic/icon in the top right corner, then click on “Manage your Google account.”
On the horizontal menu bar towards the top, click on “Personal Info.” Scroll way way down and click on “Language.” Set your “Preferred language” to whichever language you want Youtube comments to be translated into (e.g., English).
Set the “Automatically add languages” tab to OFF.
Delete any languages listed under “Other Languages” as otherwise comments in those other languages will NOT be translated (probably because YouTube foolishly assumes you already understand those languages). That’s it!
Wait for a few hours up to 2–3 days for YouTube to process your changes. After at most 2–3 days, you will then be given the option to translate any foreign comments to your preferred language! I had to wait about 2 days before it started working, but it worked (and continues to work months later)!!!