iRule required to change URL to public IP address.

I believe not as I have auto-map configured on the VIP and I can successfully login with https://1.1.1.1/en which is the external VIP IP.

Just to clarify, (SNAT) auto-map is a routing mechanism. If that wasn't working you wouldn't be getting any responses at all from the web server. Your original description stated that the URI "ends in /en". I'm now assuming that it actually starts with "/en", so the iRule would be subtly different:

when HTTP_REQUEST {                                                
     replace the Host header
    HTTP::header replace Host "1.1.1.1"     

     select a pool and rewrite URI based on URI 
    switch -glob [string tolower [HTTP::uri]] {
        "/en*" {
            pool pool_a
            HTTP::uri "/wfc/logon"
        }
        "/fr*" {
            pool pool_b
            HTTP::uri "/wfc/logon"
        }
    }
}

This should at least fix the pool selection.