I found a workaround:
$usSession = New-PSSessionOption -Culture en-US -UICulture en-US
Invoke-Command -ComputerName $serverName -SessionOption $usSession -ScriptBlock `
# Should be in en-US culture
# Should work now
I had to run commands on the real server anyway to process local paths. Not sure if you are allowed to set the local culture in the existing session, so invoke might be necessary even if you don't want to execute remotely.
Also for this feature, I suggest an alternate behaviour that the PS error level should not be "error" when localization file is missing. Perhaps we are only experiencing this error because of Set-StrictMode. I also set $ErrorActionPreference to "Stop". If the en-GB localization is not there, we should fallback to en-US without an error level causing us to break.
Same error with en-GB display language (UI culture) in de-DE region (format culture) on a en-US (MUI) Windows 10 OS.
Would be a great replacement for Windows Home Server or to avoid having to mess about with Linux for proper DNS services at home. Better than just RasPi2 would be if this could run on ANY Windows IoT supported platform. The RasPi doesn't have very good IO capabilities (regarding speed), but other boards which are also IoT supported do have SATA and/or USB3. So IoT together with Nano server could be perfect for many home users, makers, microl businesses and even special applications in large businesses.