Are you using the nightly updated builds? Cause the regular stable Firefox builds don't have Chinese, Korean, and Japanese yet.
According to an article I saw earlier, Chinese, Korean, and Japanese support was only added a few weeks ago in the nightly builds back in January 2025. When I checked prior in December 2024, there was no mention of Chinese, Korean, and Japanese support being added yet with threads spanning back to 2013 request for them to do so.
Edit: Found an article saying that Chinese, Korean, and Japanese was only finally added in version 135 of Firefox at that start of this month on February 4th. So it is a very recent thing, though strangely enough my phone has version 135 downloaded and still does not have Chinese, Korean, or Japanese available in the download list as you can see in the picture I posted in my previous comment.
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And their official features page hasn't added Chinese, Korean, and Japanese to the supported list despite the updates logs stating that they do.