Postmaster log messages are written in whatever the database_encoding is, so if you've got multiple databases with different encodings, the encoding in the log will be inconsistent.
Thanks for your answer Tom.
but...
all DBs are encoded in UTF8 (SELECT encoding FROM pg_database gives 6 for all DBs).
Is that encoding You think about?
datcollate and datctype are Polish Poland.1250 for all DBs