Does PressAndHold automatically combine region with general language?
I can’t be sure, but I suspect the answer is NO.

I finally found some pages about how “firmlinks” can be created. One said I would have to specify them in /usr/share/firmlinks. Another said that file is only for directories from Apple and users must specify them in /etc/synthetic.conf.

Neither method worked, using the syntax in the pages’ examples.

But then I learned that the file I should modify is not actually in the read-only area.  So I made the changes I wanted and tried it out.  Didn’t work!

A little more investigation revealed that Apple put an identical file in the read-only area which overrides the one in the data area.  Copied the one I had edited into the read-only area, and the pop-ups I had added worked.