this is only a guess, but The text file should be encoded as Unicode this is not really a setting you can change. You need to convert your text file encoding. Check in the save dialog in notepad and you will see the encoding drop-box and select Unicode.
For the mobi books, I guess that calibre should convert them to the right encoding, but I can't be sure.
But several person have been able to use other character sets correctly, just make sure it is encoded to Unicode.