latin extended in Dynamic Avatar

There is issue in generation of avatar picture, if display name of user contains latin extended (vietnam, etc..) characters.
For example

Phdr. ?otek ?olek, Csc.
?ofka ?íha
áron íha

I found bug in avatar.php

		foreach ( $words as $w ) {
			$text .= mb_strtoupper( mb_substr( $w, 0, 1) );
		}

It returns correct text, but I think there is issue in imagettfbbox function.
Maybe this help
https://stackoverflow.com/questions/9458317/working-with-gd-imagettftext-and-utf-8-characters

Thanks