How to fix thin and unreadable Chinese fonts after macOS 27 update?

The fact that Chinese text looks normal when the system language is English but becomes thin again when Simplified Chinese is restored is useful diagnostic information, although it does not establish the cause.

First, open Font Book >> Settings >> Advanced >> Reset Fonts >> Proceed. 

Apple says this restores the fonts originally included with macOS and moves fonts you installed to a separate folder. (Apple Support)

Restart the Mac and test Chinese text in Finder, System Settings, and TextEdit. 

If the problem remains, use Font Book >> File >> Validate Selection and File >> Resolve Duplicates to check for font problems or duplicate fonts. (Apple Support)

Also check Font Book >> Settings >> Advanced >> Show fonts managed by non-Apple utilities. 

If you use a third-party font manager or have installed additional fonts, temporarily disabling or removing the relevant third-party fonts can help determine whether they are involved. 

Apple documents that fonts installed by third-party utilities can be displayed separately in Font Book. (Apple Support)

If the problem remains after those checks, create a temporary macOS user account, select Simplified Chinese, and test the same Apple applications. 

If the Chinese rendering is normal in the new account, that would indicate that something specific to the original user account may be involved.

I would not manually replace, delete, or modify Apple's system font files. 

Apple states that system fonts cannot be removed or deactivated through Font Book. (Apple Support)

At that point, if the problem occurs in both user accounts and only when Simplified Chinese is selected, you have a much stronger basis for treating this as a macOS-level issue rather than an individual user's font configuration.

In which case filling a Bug Report directly will Apple would be appropriate 

Apple Feedback and Bug Report 

To fix the thin, unreadable, or distorted Chinese text following the recent system update, you need to adjust your system's language priority list and clear the corrupted font caches. Major updates often reset the way macOS handles CJK (Chinese, Japanese, Korean) fallback font priorities or break the local CoreText caches, forcing the system to render Chinese using ultra-thin weights or incorrect regional font substitutes. 

if this is not working you might want to give this to apple as feedback, please do so using this link

Product Feedback - Apple

I adjusted the Preferred Languages list. When I switched the system language to English, it prompted me to restart. After restarting, the system language became English (obviously), and the Chinese font appeared to be normal. Then I switched back to Simplified Chinese. After another restart, the Chinese font became thin again.

You also mentioned a step to clear corrupted font caches. How should I do that? The AI told me to run:

```

atsutil server -shutdown

atsutil databases -remove

atsutil server -ping

```

But I get this error: "ATS is not supported starting macOS 14."

Then the AI told me to run `atsutil databases -removeUser`. There was no error this time, but after restarting, the font still didn't recover.

The AI I'm using seems to have outdated training data, and Google searches don't seem to turn up how to do this on newer macOS versions either.

Could you please tell me the correct steps for clearing font caches on the current macOS version?