I tried both “en_US” and “en-US”. Neither worked. On this page it’s listed:
https://msdn.microsoft.com/en-us/library/cc233982.aspx

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Thursday, August 10, 2017 14:40
To: Burkhardt, Glenn B UTAS
Cc: pgsql-bugs@postgresql.org
Subject: [External] Re: [BUGS] BUG #14778: locale 'en-US' not recognized
glenn.burkhardt@utas.utc.com writes:
> For an unattended install with options "--mode unattended --locale
> en-US", the installation fails claiming that no such locale exists.
> However, --locale "English, United States" works. The standard
> Microsoft designation of "en-US" is not recognized.
You sure that's standard? Everyone else spells it like "en_US".
(I would certainly not put it past Microsoft to be randomly different from every other system, but pilot error seems more likely here.)
regards, tom lane