Changing interface language explicitly regardless of HTTP header

David's right on the money with HTTP::header replace:

HTTP::header replace "Accept-Language" "en"

This can be used in a request or response, but in this case you'd be replacing a request header. As for performing URI-switching logic based on the requested language:

when HTTP_REQUEST {
    switch [HTTP::header "Accept-Language" {
        "en" {
            HTTP::uri "/en/something/"
        }
        "fr" {
            HTTP::uri "/fr/something"
        }
        ...
    }
}

You mentioned a "webtop", so if this is being used with APM, replace HTTP_REQUEST with ACCESS_ACL_ALLOWED.