feat: public API, SEO and existing surfaces

internal/domain/localization/localizationRules.go

@@ -9,6 +9,8 @@ import (
 
 var socialLocalePattern = regexp.MustCompile(`^([A-Za-z]{2,3})_([A-Za-z]{2})$`)
 
+var regionPattern = regexp.MustCompile(`^[A-Za-z]{2}$`)
+
 func NormalizeSocialLocale(tag string, social *string) (*string, error) {
 	if social == nil {
 		return nil, nil
@@ -25,25 +27,38 @@ func NormalizeSocialLocale(tag string, social *string) (*string, error) {
 	return &normalized, nil
 }
 
-func SocialLocaleState(row Row) string {
+func SocialLocaleState(row Row, siteLocale *string) string {
 	if row.SocialLocale != nil {
 		return SocialLocaleSet
 	}
-	if _, region, found := strings.Cut(row.Tag, "-"); found && region != "" {
+	if EffectiveSocialLocale(row, siteLocale) != nil {
 		return SocialLocaleDerived
 	}
 	return SocialLocaleNotSet
 }
 
-func EffectiveSocialLocale(row Row) *string {
-	switch SocialLocaleState(row) {
-	case SocialLocaleSet:
+func EffectiveSocialLocale(row Row, siteLocale *string) *string {
+	if row.SocialLocale != nil {
 		return row.SocialLocale
-	case SocialLocaleDerived:
+	}
+	if _, region, found := strings.Cut(row.Tag, "-"); found && region != "" {
 		derived := strings.Replace(row.Tag, "-", "_", 1)
 		return &derived
 	}
-	return nil
+	return borrowedSocialLocale(row.Tag, siteLocale)
+}
+
+func borrowedSocialLocale(tag string, siteLocale *string) *string {
+	if siteLocale == nil {
+		return nil
+	}
+	tagLanguage, _, _ := strings.Cut(tag, "-")
+	parts := strings.Split(*siteLocale, "-")
+	if len(parts) < 2 || !strings.EqualFold(parts[0], tagLanguage) || !regionPattern.MatchString(parts[1]) {
+		return nil
+	}
+	borrowed := strings.ToLower(parts[0]) + "_" + strings.ToUpper(parts[1])
+	return &borrowed
 }
 
 func normalizeEntries(entries []Entry) ([]Entry, error) {