Hello,
It seems that I haven’t been clear enough, so I’ll try to be clearer.
– This bug doesn’t exist in v3.3.1 when it’s properly installed. I am 200% sure of that because the error occurs in a file that doesn’t exist in this version. You can donwload the zip and manually check the files. You’ll see that there is no more ‘lingotek’ folder. This is also visible in the source of this version hosted here: https://plugins.trac.www.ads-software.com/browser/polylang/tags/3.3.1
– This bug is the result of something having gone wrong during the update process. Normally WordPress completely removes all the plugin files. In your case, files from the old version are mixed with files from the new version. That’s why the plugin doesn’t work as expected.
– This bug is related to the plugin update and it is out of the plugin author control. Deactivate old plugin, remove old plugin, download new plugin, unzip files, activate new plugin, all these actions are 100% handled by WordPress, not by the plugin itself.
– I have no idea why the part of the process “remove old plugin” failed in your case. This is something I never encountered myself.
– So I hope you understand now that there will be no fix in the future because this is not a bug in Polylang.
– If you have such issue, the best to do is to manually remove the complete polylang directory and to re-install the latest version (currently 3.3.1) from the WordPress dashboard as you did the first time. This way you are sure that you’ll get Polylang properly installed without versions mix.
– There is no risk for your data in removing plugin files and re-install the new version. Remember, this is exactly what should have been done by the update process. All your data are stored in the database (that said, the best practice that I would always recommend is to do a backup before any WordPress, plugin or theme update).