I can´t register a user with ñ (español) beacause is invalid character, (i know it)
the questions are:
are there any functions that allow that?
is a server problem or a wordpress problem?
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You could hack into wp-includes/formatting.php and comment out the contents of the sanitize_user() function.
However, I don't think that's a good idea - it may compromize your site's security, and special characters can always cause trouble...
And it's never a good idea to hack into the core (maybe you can do it also with remove_action() - but I'm not very familiar with this).