Issue with English non-US Culture

I am able to use Set-Culture (Powershell as Admin) to set the Current Culture to "en-DE" which is English (Germany). However, when I run the different PS commands to view the Current Culture, I am still getting en-US. I checked my Region (Format) and Location as well.

Do I have to change the system locale as well to Germany (German) ?

This is causing an error in an application, because the datetime format is different from en-DE to en-US and causing the date to be read incorrectly.

When I Set-Culture to de-DE, everything appears to be in working order.

I make sure to run Powershell Console as Administrator, Set-Culture, close console. Open Powershell and run Get-Culture, [CultureInfo]::CurrentCulture, [CultureInfo]::CurrentUICulture and a few more to check and and still getting en-US

Note: Use of en-DE as a culture identifier - i.e., mixing language en (English) with normally unrelated region/country DE (Germany) - requires Windows 10 with release channel 1607 or later or Windows Server 2016, according to Microsoft.

However, there's a bug that prevents use of such mixed cultures, observed on Windows 10 Pro (64-bit; Version 1709, OS Build: 16299.371)

While you can successfully set such mixed-culture values with Set-Culture, subsequent sessions do not recognize it and fall back to en-US (as reflected in $PSCulture, Get-Culture and [cultureinfo]::currentCulture)

This problem has been reported on UserVoice.
Note that PowerShell Core is not affected (but it doesn't support Set-Culture).

The rest of this answer discusses persistently setting the current user's culture in general, irrespective of the bug.

Set-Culture - via the registry - sets the culture for future PowerShell sessions (only), not (also) for the current session.

Get-Culture, by contrast, only ever reports the current session's culture at session-startup time. That is, if you change the culture during a session (see below), it will not be reflected in Get-Culture.

In order to also apply the newly set culture to the current session, run the following in addition to the Set-Culture call:

[cultureinfo]::CurrentCulture = 'de-DE'

Caveat re interactive (command-line) use:

In Windows PowerShell (still as of v5.1), the active culture is reset after every command submitted; e.g.,
[cultureinfo]::CurrentCulture = 'de-DE'; Get-Date works as expected, because it is part of the same command line, but when executing just Get-Date as the next command, the current culture has reverted to the one that was current at session-startup time.
- This problem has been fixed in PowerShell Core.

This perhaps surprising asymmetry - Set-Culture only applying to future sessions, but Get-Culture reporting the current session's (startup) culture - is something that may change in future PowerShell Core versions.