In JavaScript, when you want to deal with Unicode in regular expressions, the usual solution is to give up.
The next most usual solution is to use xregexp which does happen to have the classes you seem to need :
var unicodeWord = XRegExp('^\\p{L}+$');
unicodeWord.test('Русский'); // -> true
unicodeWord.test('日本語'); // -> true
unicodeWord.test('العربية'); // -> true