feat(serverHandler): remove Content-Language response header

As language code has already been specified on `html` tag's `lang`
attribute from response page, language code from response header could
be ommitted.

This makes it possible to let frontend determines which language to
render.

Author: marjune163

src/serverHandler/page.go

@@ -71,7 +71,6 @@ func (h *handler) page(w http.ResponseWriter, r *http.Request, data *responseDat
 	header.Set("Cache-Control", "public, max-age=0")
 
 	updateTranslation(r, data)
-	header.Set("Content-Language", data.Lang)
 
 	if !needResponseBody(r.Method) {
 		w.WriteHeader(data.Status)