gistfile1.ps1

#Using Start-Process you can get the correct encoding of your program output. 
#However you have to use a temp. filename because it's the only thing Start-Process allows to. 
#I wrote this function, taking program path and arguments, and returns stdout decoded from 
#program output as UTF8 Function 

Start-ProcessClean 
{
Param( [Parameter(Mandatory=$True, Position=1)][string]$FilePath, 
       [Parameter(Mandatory=$False,Position=2)][string[]]$ArgumentList )
  
  $stdOutTempFile = [IO.Path]::GetTempFileName()
  $stdErrTempFile = [IO.Path]::GetTempFileName() 
  Start-Process $FilePath -ArgumentList $ArgumentList -NoNewWindow -Wait -RedirectStandardOutput $stdOutTempFile -RedirectStandardError $stdErrTempFile 
  $stdout = Get-Content $stdOutTempFile -Encoding UTF8
  rm $stdOutTempFile; rm $stdErrTempFile 
  return $stdout
} 

#Call it like
$out = Start-ProcessClean "program.exe" 'first_argument "argument with spaces"'