I am sure there is no technical problem but maybe gmail and hotmail are not supporting that on purpose. This kind of websites have a wide audience and should be accessible from everywhere.
Let’s imagine the user have a password in Japanese but he is on travel and go to a cyber cafe and there is no Japanese support the user won’t be able to login.
One other problem is to analyze the password complexity, it’s not so difficult to make sure the user didn’t type a common word in English but what about in Chinese/Russian/Thai. It is much more difficult to analyze the complexity of a password as you add more languages.
So in case you want your system to be accessible, it’s better to ensure that the user would be able to type his password on every kind of devices/OSes/environments, so the alpha numeric password with most common symbols(
!<>"#$%& etc..) is kind of good set of characters available everywhere.