Add comprehensive i18n localization support

- Add go-i18n package for internationalization
- Create i18n middleware to extract Accept-Language header
- Add translation files for en, es, fr, de, pt languages
- Localize all handler error messages and responses
- Add language context to all API handlers

Supported languages: English, Spanish, French, German, Portuguese

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/i18n/i18n.go

@@ -0,0 +1,86 @@
+package i18n
+
+import (
+	"embed"
+	"encoding/json"
+
+	"github.com/nicksnyder/go-i18n/v2/i18n"
+	"github.com/rs/zerolog/log"
+	"golang.org/x/text/language"
+)
+
+//go:embed translations/*.json
+var translationFS embed.FS
+
+// Bundle is the global i18n bundle
+var Bundle *i18n.Bundle
+
+// SupportedLanguages lists all supported language codes
+var SupportedLanguages = []string{"en", "es", "fr", "de", "pt"}
+
+// DefaultLanguage is the fallback language
+const DefaultLanguage = "en"
+
+// Init initializes the i18n bundle with embedded translation files
+func Init() error {
+	Bundle = i18n.NewBundle(language.English)
+	Bundle.RegisterUnmarshalFunc("json", json.Unmarshal)
+
+	// Load all translation files
+	for _, lang := range SupportedLanguages {
+		path := "translations/" + lang + ".json"
+		data, err := translationFS.ReadFile(path)
+		if err != nil {
+			log.Warn().Str("language", lang).Err(err).Msg("Failed to load translation file")
+			continue
+		}
+		Bundle.MustParseMessageFileBytes(data, path)
+		log.Info().Str("language", lang).Msg("Loaded translation file")
+	}
+
+	return nil
+}
+
+// NewLocalizer creates a new localizer for the given language tags
+func NewLocalizer(langs ...string) *i18n.Localizer {
+	return i18n.NewLocalizer(Bundle, langs...)
+}
+
+// T translates a message ID with optional template data
+func T(localizer *i18n.Localizer, messageID string, templateData map[string]interface{}) string {
+	if localizer == nil {
+		localizer = NewLocalizer(DefaultLanguage)
+	}
+
+	msg, err := localizer.Localize(&i18n.LocalizeConfig{
+		MessageID:    messageID,
+		TemplateData: templateData,
+	})
+	if err != nil {
+		// Fallback to message ID if translation not found
+		log.Debug().Str("message_id", messageID).Err(err).Msg("Translation not found")
+		return messageID
+	}
+	return msg
+}
+
+// TSimple translates a message ID without template data
+func TSimple(localizer *i18n.Localizer, messageID string) string {
+	return T(localizer, messageID, nil)
+}
+
+// MustT translates a message ID or panics
+func MustT(localizer *i18n.Localizer, messageID string, templateData map[string]interface{}) string {
+	if localizer == nil {
+		localizer = NewLocalizer(DefaultLanguage)
+	}
+
+	msg, err := localizer.Localize(&i18n.LocalizeConfig{
+		MessageID:    messageID,
+		TemplateData: templateData,
+	})
+	if err != nil {
+		panic(err)
+	}
+	return msg
+}