I have solution for you 
The powershell file must be saved with encoding “UTF-8 with BOM” and you need to use an encoding setting in the header line. Like the following example
Write-Host('<<<encoding_test:sep(9):encoding(cp437)>>>')
Write-Host('Test ÄÖÜ - Hello ä é ô')
Output on the monitoring system looks like this.
I think the most important part is the “cp437” codepage and the format of your powershell script with “UTF-8 with BOM”