Hm, very interesting. I have added all not english symbols with html codes, in guestbook, symbols seems ok, but in the error window (When one or more forms is missing) the form names is not showing few symbols properly

That is because the page isnt coded in unicode, you probably using ANSI. I had same problem with my site, just some symbols are not showing good, even with typing html code, then showing that code, or when symbol, showing ? upside-down . You can see an examle at my guestbook, when not filling few forms you get error window :
http://www.ultraguest.com/sign/1201633492