Where is the language of psql system messages defined?

I am installing postgresql 16 on a Japanese version of windows. I do not use any wsl. C:\Program Files\PostgreSQL\16\data\postgresql.conf All settings such as lc_messages in the configuration file are specified with C. The only timezone specified is Japan.

timezone = 'Asia/Tokyo'
lc_messages = C # locale for system error message
lc_monetary = C # locale for monetary formatting
lc_numeric = C # locale for number formatting
lc_time = C # locale for time formatting

No special environment variables are specified for the terminal. That is, LC_MESSAGES is not specified. If you connect to the postgresql server on localhost with psql in this state, you will see the system messages in Japanese. The red frame is in Japanese. In the same environment, if the environment variable LC_MESSAGES=C is specified in the terminal, the system messages will be in English. Note that the message is in English from the message before entering the correct password. Where is this behavior described in the postgresql help? Is there any way to make the system messages in psql English other than specifying LC_MESSAGES=C in the terminal?