fix: respect Accept-Language quality factors in language detection

lib/localization/localization.go

@@ -81,7 +81,28 @@ func (ls *LocalizationService) GetLocalizerFromRequest(r *http.Request) *i18n.Lo
 		return i18n.NewLocalizer(bundle, "en")
 	}
 	acceptLanguage := r.Header.Get("Accept-Language")
-	return i18n.NewLocalizer(ls.bundle, acceptLanguage, "en")
+
+	// Parse Accept-Language header to properly handle quality factors
+	// The language.ParseAcceptLanguage function returns tags sorted by quality
+	tags, _, err := language.ParseAcceptLanguage(acceptLanguage)
+	if err != nil || len(tags) == 0 {
+		return i18n.NewLocalizer(ls.bundle, "en")
+	}
+
+	// Convert parsed tags to strings for the localizer
+	// We include both the full tag and base language to ensure proper matching
+	langs := make([]string, 0, len(tags)*2+1)
+	for _, tag := range tags {
+		langs = append(langs, tag.String())
+		// Also add base language (e.g., "en" for "en-GB") to help matching
+		base, _ := tag.Base()
+		if base.String() != tag.String() {
+			langs = append(langs, base.String())
+		}
+	}
+	langs = append(langs, "en") // Always include English as fallback
+
+	return i18n.NewLocalizer(ls.bundle, langs...)
 }
 
 // SimpleLocalizer wraps i18n.Localizer with a more convenient API

lib/localization/localization_test.go

@@ -3,6 +3,7 @@ package localization
 import (
 	"encoding/json"
 	"fmt"
+	"net/http/httptest"
 	"sort"
 	"testing"
 
@@ -138,3 +139,40 @@ func TestComprehensiveTranslations(t *testing.T) {
 		})
 	}
 }
+
+func TestAcceptLanguageQualityFactors(t *testing.T) {
+	service := NewLocalizationService()
+
+	testCases := []struct {
+		name           string
+		acceptLanguage string
+		expectedLang   string
+	}{
+		{"simple_en", "en", "en"},
+		{"simple_de", "de", "de"},
+		{"en_GB_with_lower_priority_de", "en-GB,de-DE;q=0.5", "en"},
+		{"en_GB_only", "en-GB", "en"},
+		{"de_with_lower_priority_en", "de,en;q=0.5", "de"},
+		{"de_DE_with_lower_priority_en", "de-DE,en;q=0.5", "de"},
+		{"fr_with_lower_priority_de", "fr,de;q=0.5", "fr"},
+		{"zh_CN_regional", "zh-CN", "zh-CN"},
+		{"zh_TW_regional", "zh-TW", "zh-TW"},
+		{"pt_BR_regional", "pt-BR", "pt-BR"},
+		{"complex_header", "fr-FR,fr;q=0.9,en-US;q=0.8,en;q=0.7,de;q=0.5", "fr"},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			req := httptest.NewRequest("GET", "/", nil)
+			req.Header.Set("Accept-Language", tc.acceptLanguage)
+
+			localizer := service.GetLocalizerFromRequest(req)
+			sl := &SimpleLocalizer{Localizer: localizer}
+
+			gotLang := sl.GetLang()
+			if gotLang != tc.expectedLang {
+				t.Errorf("Accept-Language %q: expected %s, got %s", tc.acceptLanguage, tc.expectedLang, gotLang)
+			}
+		})
+	}
+}