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/middleware.go

@@ -0,0 +1,122 @@
+package i18n
+
+import (
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"github.com/nicksnyder/go-i18n/v2/i18n"
+	"golang.org/x/text/language"
+)
+
+const (
+	// LocalizerKey is the key used to store the localizer in Gin context
+	LocalizerKey = "i18n_localizer"
+	// LocaleKey is the key used to store the detected locale in Gin context
+	LocaleKey = "i18n_locale"
+)
+
+// Middleware returns a Gin middleware that detects the user's preferred language
+// from the Accept-Language header and stores a localizer in the context
+func Middleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// Get Accept-Language header
+		acceptLang := c.GetHeader("Accept-Language")
+
+		// Parse the preferred languages
+		langs := parseAcceptLanguage(acceptLang)
+
+		// Create localizer with the preferred languages
+		localizer := NewLocalizer(langs...)
+
+		// Determine the best matched locale for storage
+		locale := matchLocale(langs)
+
+		// Store in context
+		c.Set(LocalizerKey, localizer)
+		c.Set(LocaleKey, locale)
+
+		c.Next()
+	}
+}
+
+// parseAcceptLanguage parses the Accept-Language header and returns a slice of language tags
+func parseAcceptLanguage(header string) []string {
+	if header == "" {
+		return []string{DefaultLanguage}
+	}
+
+	// Parse using golang.org/x/text/language
+	tags, _, err := language.ParseAcceptLanguage(header)
+	if err != nil || len(tags) == 0 {
+		return []string{DefaultLanguage}
+	}
+
+	// Convert to string slice and normalize
+	langs := make([]string, 0, len(tags))
+	for _, tag := range tags {
+		base, _ := tag.Base()
+		lang := strings.ToLower(base.String())
+
+		// Only add supported languages
+		for _, supported := range SupportedLanguages {
+			if lang == supported {
+				langs = append(langs, lang)
+				break
+			}
+		}
+	}
+
+	// If no supported languages found, use default
+	if len(langs) == 0 {
+		return []string{DefaultLanguage}
+	}
+
+	return langs
+}
+
+// matchLocale returns the best matching locale from the provided languages
+func matchLocale(langs []string) string {
+	for _, lang := range langs {
+		for _, supported := range SupportedLanguages {
+			if lang == supported {
+				return supported
+			}
+		}
+	}
+	return DefaultLanguage
+}
+
+// GetLocalizer retrieves the localizer from the Gin context
+func GetLocalizer(c *gin.Context) *i18n.Localizer {
+	if localizer, exists := c.Get(LocalizerKey); exists {
+		if l, ok := localizer.(*i18n.Localizer); ok {
+			return l
+		}
+	}
+	return NewLocalizer(DefaultLanguage)
+}
+
+// GetLocale retrieves the detected locale from the Gin context
+func GetLocale(c *gin.Context) string {
+	if locale, exists := c.Get(LocaleKey); exists {
+		if l, ok := locale.(string); ok {
+			return l
+		}
+	}
+	return DefaultLanguage
+}
+
+// LocalizedError returns a localized error message
+func LocalizedError(c *gin.Context, messageID string, templateData map[string]interface{}) string {
+	return T(GetLocalizer(c), messageID, templateData)
+}
+
+// LocalizedMessage returns a localized message
+func LocalizedMessage(c *gin.Context, messageID string) string {
+	return TSimple(GetLocalizer(c), messageID)
+}
+
+// LocalizedMessageWithData returns a localized message with template data
+func LocalizedMessageWithData(c *gin.Context, messageID string, templateData map[string]interface{}) string {
+	return T(GetLocalizer(c), messageID, templateData)
+}