Character-encoding problem with string literal in source code

Question

$logstring = Invoke-Command -ComputerName $filesServer   -ScriptBlock {
        param(
            $logstring,
            $grp
        )

    $Klassenbuchordner = "KB " + $grp.Gruppe
    $Gruppenordner = $grp.Gruppe
    $share = $grp.Gruppe
    $path = "D:\Gruppen\$Gruppenordner"

    if ((Test-Path D:\Dozenten\01_Klassenbücher\$Klassenbuchordner) -eq $true)
    {$logstring += "Verzeichnis für Klassenbücher existiert bereits"}
    else {
        mkdir D:\Dozenten\01_Klassenbücher\$Klassenbuchordner
        $logstring += "Klassenbuchordner wurde erstellt!"
    }} -ArgumentList $logstring, $grp

My goal is to test the existence of a directory and create it on demand.

The problem is that the path contains German letters (umlauts), which aren't seen correctly by the target server.

For instance, the server receives path "D:\Dozent\01_Klassenbücher" instead of the expected "D:\Dozent\01_Klassenbücher".

How can I force proper UTF-8 encoding?