Why do my buttons keep becoming numbers?
I upgraded from 5.28 last night and ran into this same problem. I had an old version of haystack theme installed which was causing the problem (disabling the theme would bring the labels back).
You'll want to disable and uninstall haystack, delete the directory from your wp/uploads/civicrm/ext/ directory and then install the updated theme from here: https://lab.civicrm.org/extensions/haystacktheme
That fixed it for me.