Thanks @sam!
Indeed, we have this line in our container/app.yml
db_default_text_search_config: "pg_catalog.czech"
But it has been there for ages so it is a bit weird it is making problems only in this specific case now.
I found this Docker image with preconfigured PSQL witch Czech FTS, but I have no idea whether it is feasible or advisable to somehow include in the Discourse Docker install.
db_default_text_search_config: “pg_catalog.czech”