# for .Net methods, you need absolute paths
$file = [System.IO.Path]::GetFullPath([System.IO.Path]::Combine($pwd, '.\myinifile.ini'))
# get the 1251 encoding object
$cyrillicEnc = [System.Text.Encoding]::GetEncoding("windows-1251")
# read all lines in an array
$contents = [System.IO.File]::ReadAllLines($file, $cyrillicEnc)
# and loop over them using your regex
$CommentChar = ";"
$commentRegex = "^\s*([$($CommentChar -join '')].*)$"
foreach ($line in $contents) {
if ($line -match $commentRegex) { Write-Host $matches[1] }
}