I inform about a bug with language customization. More details could be found here, in forum:
When I wanted to customize the language in this version of Moodle and write the changed strings to a file, I got an error described in the forum.
The problem was in my case with no longer used themes (Essesntial, Moove), but some users describe problems with other plugins.
To solve it, a direct intervention in the MySQL database is necessary, which I do not consider safe, and I suppose that it is necessary to fix the problem...