I have the same problem on server with PHP Version 4.4.2 although the same instalation on another server works well. There are many discussion on this theme around the net. Try this one: "Can hosting provider disable localization?" here at WordPress.org.
This is what helped me:
in gettext.php changing strings
// $MAGIC1 = (int)0×950412de; //bug in PHP 5
$MAGIC1 = (int) - 1794895138;
// $MAGIC2 = (int)0xde120495; //bug
$MAGIC2 = (int) - 569244523;
$MAGIC1 = 2500072158;
$MAGIC2 = 3725722773;
Frontend is translated now (using .mo file of the theme) but admin is still in English (doesn't use .mo file from wp-includes/languages).
WP developers where are you? Is it a bug or something? It seems that many people suffer from this problem. WP is great. Translated one is even better. Can anybody answer?