You have to make changes to your PHP.ini file to set the `mbstring.internal_encoding` to the value UTF-8. If you need help doing this you will need to ask your ISP for that. For my ISP, at least, there is a management screen that lets me edit the PHP.ini file.
This problem may not be an issue for you, however. If you just run everything in English with no accented characters terer will be no problem (as the message says.)