What happens when you hit the back button in the browser should not be affected by anything the script does (nor can it be influenced by it). That would be a purely client-side (browser) issue.

I am not sure how this has anything to do with the characters being UTF-8 or not. Does it display non-UTF8 characters correctly? In any case, you have to make sure that you have the character set correctly specified in the HTTP headers and/or the HTML file itself. Different browsers have different logic to figure out the character set, and if the problem you describe only happens with non-Ascii characters, it could be caused by the browser getting things mixed up.