I need an iRule that will search the HTTP::path for "/en/' change it to "/en-us/" and leave the rest of the path, query string intact

Try this:

when HTTP_REQUEST {
    if { [string tolower [HTTP::uri]] starts_with "/en/" } {
        set uri [string map -nocase {"/en/" "/en-us/"} [HTTP::uri]]
        HTTP::redirect "http://[HTTP::host]${uri}"
    }
}