OK, after a bit more work, I got it more or less under control. So here's a summary of issues/findings in case it's useful:
1. Momentarily choosing one of the default Twenty WordPress templates during the import can avoid problems.
2. Making sure that there's a high memory limit set on wp-config.php too.
3. For some reason my site didn't need the generated .htaccess rules and actually works perfectly without them and not at all with them.
4. It's necessary to save the permalinks.
5. Widget translations are not imported.
6. Some menu translations are not imported. It's necessary to synchronize menu items for translated menus to work at all.
7. Post categories are not preserved on translated posts, which can be a huge task.
I've managed to more or less fix my problems one by one, although the post categories really needs some automated solution.
I'm glad to read that you managed to fix most of the problems after the import. Thanks a lot for the detailed input that you added here. Please let me know if any further help is needed with this issue.
You can try WPML's troubleshooting section to see if it solved the issue.
1. Take a backup of the DB of your site.
2. Go to WPML -> Support -> Troubleshooting (link)
3. Once on the troubleshooting screen press the following buttons and check if it helps. Press each button at a time and wait for the process to complete.
- 'Synchronize posts taxonomies' for your [post types].
- 'Clear the cache in WPML'
- 'Remove ghost entries from the translation tables'
- 'Fix element_type collation'
- 'Fix WPML tables collation'
- 'Set language information'
- 'Assign translation status to duplicated content'
- 'Fix terms count'
- 'Fix post type assignment for translations'
- 'Cleanup and optimize string tables'
*** Please make a backup of your database before using this.***
That fixed the categories problem, thank you! I don't know if there's an automated fix for widgets but it was easy enough to do it manually in this case.
