$Lang="en-US"

# Make sure English (US) language pack is installed (UI needs "Language.Basic")
Add-WindowsCapability -Online -Name "Language.Basic~~~$Lang~0.0.1.0"

# Add en-US to your existing language list without removing anything (keeps keyboards/layouts)
$list = Get-WinUserLanguageList
if ($list.LanguageTag -notcontains $Lang) { $list.Add($Lang) }

# Put en-US first (Windows uses first entry as primary for UI/input preferences)
$en = $list | Where-Object LanguageTag -eq $Lang
$rest = $list | Where-Object LanguageTag -ne $Lang
$list = New-Object System.Collections.Generic.List[Microsoft.InternationalSettings.Commands.WinUserLanguage]
$list.Add($en); $rest | ForEach-Object { $list.Add($_) }

Set-WinUserLanguageList $list -Force

# Force Windows display language override
Set-WinUILanguageOverride -Language $Lang

"Done. Sign out/in (or reboot) to apply."