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/middleware_test.go

@@ -0,0 +1,99 @@
+package i18n
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+func TestLanguageMiddleware(t *testing.T) {
+	translator, _ := NewTranslator()
+	middleware := LanguageMiddleware(translator)
+
+	tests := []struct {
+		name           string
+		acceptLanguage string
+		expectedLang   language.Tag
+	}{
+		{
+			name:           "English header",
+			acceptLanguage: "en-US",
+			expectedLang:   language.English,
+		},
+		{
+			name:           "Spanish header",
+			acceptLanguage: "es-ES",
+			expectedLang:   language.Spanish,
+		},
+		{
+			name:           "No header defaults to English",
+			acceptLanguage: "",
+			expectedLang:   language.English,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var capturedLang language.Tag
+			handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				capturedLang = GetLanguageFromContext(r.Context())
+				w.WriteHeader(http.StatusOK)
+			})
+
+			wrappedHandler := middleware(handler)
+
+			req := httptest.NewRequest("GET", "/test", nil)
+			if tt.acceptLanguage != "" {
+				req.Header.Set("Accept-Language", tt.acceptLanguage)
+			}
+
+			w := httptest.NewRecorder()
+			wrappedHandler.ServeHTTP(w, req)
+
+			if capturedLang != tt.expectedLang {
+				t.Errorf("Expected language %v, got %v", tt.expectedLang, capturedLang)
+			}
+		})
+	}
+}
+
+func TestGetLanguageFromContext(t *testing.T) {
+	tests := []struct {
+		name     string
+		ctx      context.Context
+		expected language.Tag
+	}{
+		{
+			name:     "Context with English",
+			ctx:      context.WithValue(context.Background(), languageKey, language.English),
+			expected: language.English,
+		},
+		{
+			name:     "Context with Spanish",
+			ctx:      context.WithValue(context.Background(), languageKey, language.Spanish),
+			expected: language.Spanish,
+		},
+		{
+			name:     "Context without language defaults to English",
+			ctx:      context.Background(),
+			expected: language.English,
+		},
+		{
+			name:     "Context with wrong value type defaults to English",
+			ctx:      context.WithValue(context.Background(), languageKey, "invalid"),
+			expected: language.English,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := GetLanguageFromContext(tt.ctx)
+			if result != tt.expected {
+				t.Errorf("Expected %v, got %v", tt.expected, result)
+			}
+		})
+	}
+}