LanguageTag     : en-US
Autonym         : English (United States)
EnglishName     : English
LocalizedName   : English (United States)
ScriptName      : Latin
InputMethodTips : {0409:00000409}
Spellchecking   : True
Handwriting     : False
Actual behavior
Get-WinUserLanguageList: Cannot marshal 'parameter #2': Invalid managed/unmanaged type combination.
Environment data
> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      7.1.0
PSEdition                      Core
GitCommitId                    7.1.0
OS                             Microsoft Windows 10.0.19042
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0
Exception             :
    Type       : System.Runtime.InteropServices.MarshalDirectiveException
    TargetSite :
        Name          : GetUserLanguages
        DeclaringType : Microsoft.InternationalSettings.Commands.LPAPIWrapper,
Microsoft.InternationalSettings.Commands, Version=3.0.0.0, Culture=neutral,
PublicKeyToken=31bf3856ad364e35
        MemberType    : Method
        Module        : Microsoft.InternationalSettings.Commands.dll
    StackTrace :
   at Microsoft.InternationalSettings.Commands.LPAPIWrapper.GetUserLanguages(Char Delimiter, String&
UserLanguages)
   at Microsoft.InternationalSettings.Commands.GetWinUserLanguageListCommand.ProcessRecord()
   at System.Management.Automation.CommandProcessor.ProcessRecord()
    Message    : Cannot marshal 'parameter #2': Invalid managed/unmanaged type combination.
    Source     : Microsoft.InternationalSettings.Commands
    HResult    : -2146233035
CategoryInfo          : NotSpecified: (:) [Get-WinUserLanguageList], MarshalDirectiveException
FullyQualifiedErrorId : System.Runtime.InteropServices.MarshalDirectiveException,Microsoft.Intern
ationalSettings.Commands.GetWinUserLanguageListCommand
InvocationInfo        :
    MyCommand        : Get-WinUserLanguageList
    ScriptLineNumber : 1
    OffsetInLine     : 1
    HistoryId        : 3
    Line             : Get-WinUserLanguageList
    PositionMessage  : At line:1 char:1
                       + Get-WinUserLanguageList
                       + ~~~~~~~~~~~~~~~~~~~~~~~
    InvocationName   : Get-WinUserLanguageList
    CommandOrigin    : Internal
ScriptStackTrace      : at <ScriptBlock>, <No file>: line 1
          
Have you tried this to import Windows PowerShell module in WindowsPowerShell mode which is available as a parameter for Import-Module cmdlet? Had same error and had to run this following lines first.
Import-Module -Name International -UseWindowsPowerShell -Verbose
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Have you tried this to import Windows PowerShell module in WindowsPowerShell mode which is available as a parameter for Import-Module cmdlet? Had same error and had to run this following lines first.
Import-Module -Name International -UseWindowsPowerShell -Verbose
Thanks, @stewardjornsen! It executed successfully after running the import.
          
Is it self-evident that  Set-WinUserLanguageList ... and New-WinUserLanguageList en-GB also don't work, or should I raise a separate issue?
This is my code that worked (and still works) in PS5 but doesn't in PS7:
$ll= New-WinUserLanguageList en-GB
$cafeencounterAppleM7903="0809:A0000809" 
$enUSIntl="0809:00020409"; 
$ll[0].InputMethodTips.Add($cafeencounterAppleM7903)
$ll[0].InputMethodTips.Add($enUSIntl)
$ll[0].InputMethodTips.Remove("0809:00000809") | out-null
$ll.Add("el")
$global:ChrisKeyboardList=$ll
function keyboard(){ Set-WinUserLanguageList $global:ChrisKeyboardList -Force}
With errors:
New-WinUserLanguageList: Cannot marshal 'parameter #1': Invalid managed/unmanaged type combination.
Set-WinUserLanguageList: Cannot bind parameter 'LanguageList'. Cannot convert the "Microsoft.InternationalSettings.Commands.WinUserLanguage" value of type "Deserialized.Microsoft.InternationalSettings.Commands.WinUserLanguage" to type "Microsoft.InternationalSettings.Commands.WinUserLanguage".