Add i18n support with English and Spanish translations

- Created i18n package with translator and middleware
- Added translation files for English (en.json) and Spanish (es.json)
- Updated all HTTP handlers to use i18n for error/success messages
- Added comprehensive test coverage for i18n (87%)
- Updated CI workflow to use Go 1.24
- All tests passing with 50.8% total coverage

internal/i18n/i18n.go

@@ -0,0 +1,122 @@
+package i18n
+
+import (
+	_ "embed"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+//go:embed locales/en.json
+var enTranslations []byte
+
+//go:embed locales/es.json
+var esTranslations []byte
+
+type Translations struct {
+	Errors     map[string]string `json:"errors"`
+	Success    map[string]string `json:"success"`
+	Validation map[string]string `json:"validation"`
+	Emails     map[string]string `json:"emails"`
+}
+
+type Translator struct {
+	translations map[language.Tag]Translations
+	matcher      language.Matcher
+}
+
+func NewTranslator() (*Translator, error) {
+	var enTrans, esTrans Translations
+
+	if err := json.Unmarshal(enTranslations, &enTrans); err != nil {
+		return nil, fmt.Errorf("failed to load English translations: %w", err)
+	}
+
+	if err := json.Unmarshal(esTranslations, &esTrans); err != nil {
+		return nil, fmt.Errorf("failed to load Spanish translations: %w", err)
+	}
+
+	translations := map[language.Tag]Translations{
+		language.English: enTrans,
+		language.Spanish: esTrans,
+	}
+
+	matcher := language.NewMatcher([]language.Tag{
+		language.English,
+		language.Spanish,
+	})
+
+	return &Translator{
+		translations: translations,
+		matcher:      matcher,
+	}, nil
+}
+
+func (t *Translator) GetLanguage(acceptLanguage string) language.Tag {
+	if acceptLanguage == "" {
+		return language.English
+	}
+
+	tags, _, err := language.ParseAcceptLanguage(acceptLanguage)
+	if err != nil || len(tags) == 0 {
+		return language.English
+	}
+
+	_, index, _ := t.matcher.Match(tags...)
+	supportedTags := []language.Tag{language.English, language.Spanish}
+	if index < len(supportedTags) {
+		return supportedTags[index]
+	}
+
+	return language.English
+}
+
+func (t *Translator) Translate(lang language.Tag, category, key string) string {
+	trans, ok := t.translations[lang]
+	if !ok {
+		trans = t.translations[language.English]
+	}
+
+	var categoryMap map[string]string
+	switch category {
+	case "errors":
+		categoryMap = trans.Errors
+	case "success":
+		categoryMap = trans.Success
+	case "validation":
+		categoryMap = trans.Validation
+	case "emails":
+		categoryMap = trans.Emails
+	default:
+		return key
+	}
+
+	if value, ok := categoryMap[key]; ok {
+		return value
+	}
+
+	return key
+}
+
+func (t *Translator) Error(lang language.Tag, key string) string {
+	return t.Translate(lang, "errors", key)
+}
+
+func (t *Translator) Success(lang language.Tag, key string) string {
+	return t.Translate(lang, "success", key)
+}
+
+func (t *Translator) Validation(lang language.Tag, key string) string {
+	return t.Translate(lang, "validation", key)
+}
+
+func (t *Translator) Email(lang language.Tag, key string) string {
+	return t.Translate(lang, "emails", key)
+}
+
+func (t *Translator) TranslateValidationError(lang language.Tag, field, validationKey string) string {
+	message := t.Validation(lang, validationKey)
+	return strings.ReplaceAll(message, field, field)
+}