Custom taxonomy broken after upgrade

On the website I worked on, I just upgraded all plugins, including Sublanguage. After this, I noticed that my custom taxonomy no longer works. I tried disabling Sublanguage, and then the taxonomy works again, so I suppose the problem is with Sublanguage.

After enabling Sublanguage again, I checked that using untranslated taxonomies works with Sublanguage again, so it seems that it only chokes up when a taxonomy is translated.

Some possibly helpful info:
* The plugin in question is https://github.com/rimas-kudelis/wp-issue-manager
* Originally the taxonomy is called journal, and this is translated into Lithuanian as zurnalas
* Original journal names are Lithuanian, but are translated into English

So
…/en/journal/lithuanian-name/ works
…/lt/journal/lithuanian-name/ works
…/en/journal/english-name/ doesn’t work
…/lt/zurnalas/lithuanian-name/ doesn’t work.

Maxime, do you have any ideas?