MIM Portal Search for Umlaute/Special Characters (Ä,ä,Ö etc) with Powershell

Hello everyone

I am quite new in Powershell and MIM. I am trying too find groups which contains Umlaute like ((Ä,ä,Ö etc).

When I execute following code will output every group that either contains "ä" but also the regular "a".

 if(@(Get-PSSnapin | ? { $_.Name -eq "FIMAutomation" } ).Count -eq 0)            
 {            
     Add-PSSnapin FIMAutomation;            
 }            
$groups = Export-FIMConfig -customConfig "/Group[contains(DisplayName, 'ä')] " -Uri "http://localhost:5725"  -OnlyBaseResources            
foreach ($group in $groups)            
{            
    $x = (($group.ResourceManagementObject.ResourceManagementAttributes | Where-Object {$_.AttributeName -eq "DisplayName"}).Value)
    Write-Host "$x"            
}

What do I have to change in my script to get only the groups that conatiain umlaute?

Thanks in advance!