All my SS sites are multilingual, I know that SS plays well with special chars, but not always: try to make and if statement in a template where the value to test for is not pure ASCII, for example:
<% if Country == EspaÃ±a %>
<% end_if %>
This do not work.
Anyway, I hope you'll find a solution for your problem.
Just so other people do not get the wrong idea - User Forms is compatible with SilverStripe 2.3.3. Forms in multiple languages do not work. Multilingual Support is going to be a feature of the next release.