An example Go web app which prints localized messages

main.go

package main

import (
	"context"
	"html"
	"log"
	"net/http"

	"golang.org/x/text/language"
	"golang.org/x/text/message"
)

func main() {
	http.HandleFunc("/", withMessagePrinter(func(w http.ResponseWriter, r *http.Request) {
		p := getMessagePrinter(r)
		p.Fprintf(w, "Hello, %[1]q", html.EscapeString(r.URL.Path))
	}))

	log.Fatal(http.ListenAndServe(":8080", nil))
}

func withMessagePrinter(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		t, _, _ := language.ParseAcceptLanguage(r.Header.Get("Accept-Language"))
		tag, _, _ := matcher.Match(t...)
		p := message.NewPrinter(tag)
		ctx := context.WithValue(context.Background(), messagePrinterKey, p)
		next.ServeHTTP(w, r.WithContext(ctx))
	}
}

func getMessagePrinter(r *http.Request) *message.Printer {
	return r.Context().Value(messagePrinterKey).(*message.Printer)
}

// NOTE: This code is based on
// https://github.com/golang/proposal/blob/master/design/12750-localization.md#example-statically-linked-package
// and the example at
// https://godoc.org/golang.org/x/text/language#ParseAcceptLanguage

func init() {
	for _, e := range entries {
		for _, t := range e.entry {
			message.SetString(e.lang, t.key, t.value)
		}
	}

	langs := make([]language.Tag, 0, len(entries))
	langs = append(langs, language.English)
	for _, e := range entries {
		langs = append(langs, e.lang)
	}
	matcher = language.NewMatcher(langs)
}

type entry struct {
	key   string
	value string
}

var entries = []struct {
	lang  language.Tag
	entry []entry
}{
	{
		language.Japanese, []entry{
			{"Hello, %[1]q", "こんにちは、%[1]q"},
		},
	},
}

var matcher language.Matcher

type contextKey int

const (
	messagePrinterKey contextKey = iota + 1
)

Run the server.

go run main.go

An example session using curl.

$ curl localhost:8080          
Hello, "/"
$ curl -H 'Accept-Language: ja' localhost:8080
こんにちは、"/"