I've made a few changes to this translation and it should work now, even with UTF-8:
$phrase['sign_guestbook'] = "Írok a Vendégkönyvbe";
$phrase['view_guestbook'] = "Megnézem a Vendégkönyvet";
$phrase['return_to'] = "Vissza";
$phrase['name'] = "Név";
$phrase['date'] = "Dátum";
$phrase['email'] = "E-mail";
$phrase['location'] = "Lakhely";
$phrase['message'] = "Üzenet";
$phrase['private_message'] = "Privát Üzenet";
$phrase['added'] = "Hozzáad";
$phrase['page'] = "Oldal";
$phrase['of'] = "ból";
$phrase['first'] = "Els?";
$phrase['next'] = "Következ?";
$phrase['previous'] = "El?z?";
$phrase['last'] = "Utolsó";
$phrase['private'] = "Privát";
$phrase['characters_left'] = "Karakter van hátra";
$phrase['smilies'] = "Hangulatjelek";
$phrase['verification_code'] = "Írd be az ellen?rz? kódot";
$phrase['post_message'] = "Üzenet Írása";
$phrase['preview'] = "Megnéz";
$phrase['js_required_fields'] = "Ki kell t\u00F6ltened az al\u00C1bbi ter\u00FCleteket.";
$phrase['no_messages'] = "Nincs üzenet a Vendégkönyvben.";
Please let me know if this language still does not work.
UPDATE: It should work EXCEPT for where there are question marks (?). I'll look at this again... 
I need some help with these ones:
$phrase['first'] = "Els?";
$phrase['next'] = "Következ?";
$phrase['previous'] = "El?z?";
$phrase['verification_code'] = "Írd be az ellen?rz? kódot";