PostgreSQL: Warning: Console code page (437) differs from Windows code page (1252)

From the psql documentation:

psql is built as a “console application”. Since the Windows console
windows use a different encoding than the rest of the system, you must
take special care when using 8-bit characters within psql. If psql
detects a problematic console code page, it will warn you at startup.

To change the console code page, two things are necessary:
Set the code page by entering cmd.exe /c chcp 1252. (1252 is a code
page that is appropriate for German; replace it with your value.) If
you are using Cygwin, you can put this command in /etc/profile.

So to remove that warning you need to execute chcp 1252 before you enterpsql. Using chcp without parameters gives you the current codepage.