I don't think language packs for servers are available for download in the same way that they are for desktops. I'm answering only because you've added "PowerShell" and "Windows Server PowerShell" tags to your question. Someone more familiar with the management/maintenance of Windows may give you some way to get that ISO,
Once you have the necessary ISO, this should get you started: https://umbhost.net/blog/2020/03/how-to-install-an-additional-language-pack-windows-server-2019
Here's a link to the command line options for the lksetup.exe: https://pen2.com/cmd/lpksetup/