finally i solved with this:
@users = User.find(:all, :include => :langs)
and on wiew modified with:
<td><%=h item.langs.find_by_langcode(I18n.default_locale.to_s).title %></td> <td><%=h item.langs.find_by_langcode(I18n.default_locale.to_s).langcode %></td>
this solution work, i hope right also in terms of performances