<p>In Python I could've converted it to Unicode and do '(?u)^[\w ]+$' regex search, but PHP doesn't seem to understand international \w, or does it?</p>

Although I haven't tested myself, looking at
http://us3.php.net/manual/en/reference.pcre.pattern.syntax.php
suggests the following: '/^[\p{L} ]+$/u' would work - the \p{L} will match any unicode letter.  Additionally, you can apparently write this without the curly brackets - '/^[\pL ]+$/u'.