Windows has System and User language settings. They are independed.
Use dism to get a list of all installed languages.
D:\> dism /online /Get-Intl Deployment Image Servicing and Management tool Version: 10.0.19041.572 Image Version: 10.0.19041.630 Reporting online international settings. Default system UI language : en-US System locale : de-DE Default time zone : W. Europe Standard Time Active keyboard(s) : 0407:00000407 Keyboard layered driver : PC/AT Enhanced Keyboard (101/102-Key) Installed language(s): de-DE Type : Partially localized language, MUI type. Fallback Languages en-US Installed language(s): en-US Type : Fully localized language. The operation completed successfully.
To set the Systemlanguage use the Set-WinSystemLocale coammnd-let. For example
PS D:\> Set-WinSystemLocale de-DE
Set the language for the User
PS D:\> Set-WinUserLanguageList de-DE -Force PS D:\> Set-WinUILanguageOverride -Language de-DE
If want to install additional languages download the Language pack DVD and mount it. For example to F:\
PS D:\> Add-WindowsPackage -PackagePath F:\x64\langpacks\Microsoft-Windows-Client-language-Pack_x64_es-es.cab -Online