Currently, the arrays extension does not properly handle multi-byte characters (though this may be a shortcoming of regex, I don't know):
Character: ナ Size: {{ #len: ナ }} Array size: {{ #arraydefine: array | ナ | // }}{{ #arraysize: array }}
results in
Character: ナ Size: 1 Array size: 5
Note that attempting to use #arrayprint to view the array results in the parse of the entire page's contents being aborted.