Back-end Translation problem

Hi,

I just started the translation of the back-end to Greek language but when i try the new gr.php selected in the program language the results text is everywhere: ����� ������
(question mark in black rhombus) = encoding problem

i added to first line of gr.php the
<?php header('Content-type: text/html; charset=utf-8'); ?>
but no changes...

any idea on this how to fix?
thanks in advance,