In German I use this:
#Include <String.au3>
IniWrite("umlauts.ini", "Umlauts", "Example1", _StringToHex("Ich genieße öfter üblen Käse."))
$Result = _HexToString(IniRead("umlauts.ini", "Umlauts", "Example1", ""))
msgbox(64,"Umlauts in INI", $Result)
Exit
Works perfectly. It should also work with Russian, Thai etc. chars. The only "handicap" is: you cannot read the values directly in the INI file.