I'm not sure if I am following the instructions correctly, but...

I start Blender in English. I select File/Open and I see a display like this:

I go straight into Preferences (without restarting Blender) and change my language to Chinese. I then select File / Open and see this:

This seems right. And I get the same result if I close and restart the program at any point. Am I missing a step or not understanding the complaint?

Okay, I have seen it screw up, but in fairly narrow circumstance...

Working: Within a single blender session, so not restarting at any time, I can switch between English and Chinese and back again any number of times, and each time will see the correct System List.

Working: I am in Chinese and then change language to English, THEN file/open I will see the System list correctly. Then close blender and restart and I still see the correct System List names

Working: I am in English and then change language to Chinese, but then immediately close blender. When I open it again the System List is properly in Chinese.

Broken: I am in Chinese and then change language to English, but then immediately close blender. When I open it again the System List is Chinese until I restart.

And while debugging I have been unable to see the issue - it works correctly every time.

I checked this in debug build, and I was able to reproduce.

You should not be calling IFACE_("Pictures") in fsmenu_read_system this will cause the msgid to be translated, and when you switch languages it will use the translated text as msgid, come up empty and stick with the previous translation.

fsmenu_add_windows_folder(fsmenu,
                          FS_CATEGORY_SYSTEM_BOOKMARKS,
                          &FOLDERID_Pictures,
                          "Pictures",
                          ICON_FILE_IMAGE,
                          FS_INSERT_LAST);

makes it work for me.