Allow characters in username

[Hypersoul]

Hello,
as I'll run vshare in a non english site, so most of my visitors will signup with names related to my language. I tried signing up using these characters but I got error that these characters are not allowed.
my question: instead of allowing characters, can I simply just make disallow characters and permit all others?
if yes ... how to do it please?
Thank you

[Hypersoul]

b.t.w. I'm using UTF-8 character set.
Thank you