I did this:
- full restore from my backup
- repaired all the database tables (except for those that don't support repairing)
- reset WordPress admin GUI language to English (United States)
- even reset my custom date format settings
- set default Polylang language to English (en_US)
- deactivated Polylang plugin
- repaired all the database tables again (except for those that don't support repairing)
...and:
- first time - manually upgraded Polylang plugin from version 1.1.6 to 1.2
- second time - run automatic Polylang plugin update from version 1.1.6 to 1.5.4
in both cases the result is still the same - there is no runtime error or warning in debug log during the update process, everything looks good except that Polylang plugin doesn't migrate its data.
may I send you my database dump so that you can see by yourself?