Pure ASCII is all that's allowed in HTTP headers. But, as far as HTTP is concerned, anything goes in the request body of a POST. The headers and body are always separated by a blank line. A set of headers will normally identify the format of the content/body. Responses work the same way. However, HTML has some additional rules regarding what normally goes in a POST.
EDIT: Sorry, I missed the word 'GET' in your title. Might be nice to duplicate that in the body of your question.
At any rate, I believe I am correct in saying ONLY ASCII (ANSI X3.4-1986) is allowed in the headers of any HTTP request, GET or POST. So no, ISO-8859-2 requests are not strictly valid HTTP. That said, there's probably a way to escape the desired special characters in the query string if that's what you're really asking for here.
SOURCE: https://www.rfc-editor.org/rfc/rfc2616