La guida completa all'internazionalizzazione di Go

package main

import (
    "encoding/json"
    "fmt"
    "github.com/nicksnyder/go-i18n/v2/i18n"
    "golang.org/x/text/language"
)

func main() {
    // 1. Create a bundle with a default language
    bundle := i18n.NewBundle(language.English)
    // 2. Register the unmarshal function for your file format
    bundle.RegisterUnmarshalFunc("json", json.Unmarshal)
    // 3. Load message files
    bundle.MustLoadMessageFile("locales/en.json")
    bundle.MustLoadMessageFile("locales/ja.json")
    bundle.MustLoadMessageFile("locales/de.json")
    // 4. Create a localizer and localize a message
    localizer := i18n.NewLocalizer(bundle, "ja")
    msg := localizer.MustLocalize(&i18n.LocalizeConfig{
        MessageID: "HelloWorld",
    })
    fmt.Println(msg) // "こんにちは、世界！"
}

// Simple message
msg := localizer.MustLocalize(&i18n.LocalizeConfig{
    MessageID: "HelloWorld",
})

// Message with template data
msg := localizer.MustLocalize(&i18n.LocalizeConfig{
    MessageID: "Greeting",
    TemplateData: map[string]interface{}{
        "Name": "Alice",
    },
})
// "Hello, Alice!" (en) or "こんにちは、Aliceさん！" (ja)

// Plural + template data
msg := localizer.MustLocalize(&i18n.LocalizeConfig{
    MessageID: "ItemCount",
    PluralCount: 5,
    TemplateData: map[string]interface{}{
        "Count": 5,
    },
})
// "5 items" (en) or "5個のアイテム" (ja)

// Combined: plurals + multiple variables
msg := localizer.MustLocalize(&i18n.LocalizeConfig{
    MessageID: "WelcomeBack",
    TemplateData: map[string]interface{}{
        "Name":  "Alice",
        "Count": 3,
    },
})

func handler(w http.ResponseWriter, r *http.Request) {
    // Accept-Language: ja,en;q=0.9,de;q=0.8
    accept := r.Header.Get("Accept-Language")

    // NewLocalizer accepts multiple languages — first match wins
    localizer := i18n.NewLocalizer(bundle, accept)

    msg := localizer.MustLocalize(&i18n.LocalizeConfig{
        MessageID: "HelloWorld",
    })

    w.Header().Set("Content-Type", "text/plain; charset=utf-8")
    w.Write([]byte(msg))
}

// English: 2 forms (one, other)
{
  "ItemCount": {
    "one": "{{.Count}} item",
    "other": "{{.Count}} items"
  }
}

// Arabic: 6 forms (zero, one, two, few, many, other)
{
  "ItemCount": {
    "zero": "لا عناصر",
    "one": "عنصر واحد",
    "two": "عنصران",
    "few": "{{.Count}} عناصر",
    "many": "{{.Count}} عنصرًا",
    "other": "{{.Count}} عنصر"
  }
}

// Japanese: 1 form (other)
{
  "ItemCount": {
    "other": "{{.Count}}個のアイテム"
  }
}

// go-i18n selects the correct plural form based on PluralCount
localizer := i18n.NewLocalizer(bundle, "ar")

msg := localizer.MustLocalize(&i18n.LocalizeConfig{
    MessageID:   "ItemCount",
    PluralCount: 3,
    TemplateData: map[string]interface{}{
        "Count": 3,
    },
})
// Arabic "few" form: "3 عناصر"

msg = localizer.MustLocalize(&i18n.LocalizeConfig{
    MessageID:   "ItemCount",
    PluralCount: 1,
    TemplateData: map[string]interface{}{
        "Count": 1,
    },
})
// Arabic "one" form: "عنصر واحد"

// detectLocale resolves the user's preferred language.
// Priority: query param > cookie > Accept-Language header > default
func detectLocale(r *http.Request, matcher language.Matcher) string {
    // 1. Explicit query parameter: ?lang=ja
    if lang := r.URL.Query().Get("lang"); lang != "" {
        tag, _, _ := matcher.Match(language.Make(lang))
        return tag.String()
    }
    // 2. Cookie from previous selection
    if cookie, err := r.Cookie("lang"); err == nil {
        tag, _, _ := matcher.Match(language.Make(cookie.Value))
        return tag.String()
    }
    // 3. Accept-Language header
    accept := r.Header.Get("Accept-Language")
    if accept != "" {
        tags, _, _ := language.ParseAcceptLanguage(accept)
        if len(tags) > 0 {
            tag, _, _ := matcher.Match(tags...)
            return tag.String()
        }
    }
    return "en" // 4. Default
}

// Usage:
matcher := language.NewMatcher([]language.Tag{
    language.English, language.Japanese,
    language.German, language.Spanish,
})

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    locale := detectLocale(r, matcher)
    localizer := i18n.NewLocalizer(bundle, locale)
    // ... use localizer
})

go get github.com/i18n-agent/go-locale-chain

package main

import (
    "encoding/json"
    "fmt"

    "github.com/nicksnyder/go-i18n/v2/i18n"
    localechain "github.com/i18n-agent/go-locale-chain"
    "golang.org/x/text/language"
)

func main() {
    // 1. Configure fallback chains (call once at startup)
    localechain.Configure()

    // 2. Set up go-i18n bundle as usual
    bundle := i18n.NewBundle(language.English)
    bundle.RegisterUnmarshalFunc("json", json.Unmarshal)
    bundle.MustLoadMessageFile("locales/en.json")
    bundle.MustLoadMessageFile("locales/pt.json")
    bundle.MustLoadMessageFile("locales/pt-PT.json")
    bundle.MustLoadMessageFile("locales/pt-BR.json")

    // 3. Resolve with per-key fallback
    result, _ := localechain.ResolveWithLoader("pt-BR", func(locale string) (map[string]string, error) {
        localizer := i18n.NewLocalizer(bundle, locale)
        messages := make(map[string]string)
        for _, id := range []string{"hello", "goodbye", "thanks"} {
            msg, err := localizer.Localize(&i18n.LocalizeConfig{MessageID: id})
            if err == nil {
                messages[id] = msg
            }
        }
        return messages, nil
    })

    fmt.Println(result["hello"])   // "Olá (BR)"   — from pt-BR
    fmt.Println(result["goodbye"]) // "Adeus (PT)" — fallback to pt-PT
    fmt.Println(result["thanks"])  // "Obrigado"   — fallback to pt
}

// Standalone: zero external dependencies, works with any format
localechain.Configure()

result, _ := localechain.ResolveWithLoader("es-MX", func(locale string) (map[string]string, error) {
    data, err := os.ReadFile(fmt.Sprintf("locales/%s.json", locale))
    if err != nil {
        return nil, err // Locale file doesn't exist — skip
    }
    var msgs map[string]string
    json.Unmarshal(data, &msgs)
    return msgs, nil
})
// es-MX -> es-419 -> es: each key resolved from most specific locale

# In your IDE, ask your AI assistant:
> Translate locales/en.json to German, Japanese, and Spanish

✓ locales/de.json created (1.2s)
✓ locales/ja.json created (1.5s)
✓ locales/es.json created (1.1s)

# Or use the CLI in CI/CD:
npx i18n-agent translate locales/en.json --lang de,ja,es

my-go-app/
├── locales/
│   ├── en.json           # Source language (English)
│   ├── de.json           # German translations
│   ├── ja.json           # Japanese translations
│   ├── es.json           # Spanish translations
│   ├── pt.json           # Portuguese (base)
│   ├── pt-PT.json        # Portuguese (Portugal)
│   └── pt-BR.json        # Portuguese (Brazil)
├── i18n/
│   ├── bundle.go         # Bundle initialization
│   ├── detect.go         # Locale detection logic
│   └── middleware.go      # HTTP middleware for locale
├── main.go
├── go.mod
└── go.sum