From 2c5c2b715e38cba001574abd37d76a0f07620fbe Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Thu, 2 Apr 2026 03:11:15 +0000
Subject: [PATCH] fix(i18n): normalise underscore language tags

Co-Authored-By: Virgil <virgil@lethean.io>
---
 i18n_test.go     | 12 ++++++++++++
 language.go      |  1 +
 language_test.go |  7 +++++++
 localise.go      | 12 ++++++++++--
 localise_test.go |  1 +
 service.go       |  1 +
 6 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/i18n_test.go b/i18n_test.go
index f9fd9b3..0b41012 100644
--- a/i18n_test.go
+++ b/i18n_test.go
@@ -75,6 +75,18 @@ func TestSetLanguage_Good(t *testing.T) {
 	assert.Contains(t, CurrentLanguage(), "en")
 }
 
+func TestSetLanguage_Good_UnderscoreTag(t *testing.T) {
+	svc, err := New()
+	require.NoError(t, err)
+	_ = Init()
+	SetDefault(svc)
+
+	err = SetLanguage("fr_CA")
+	assert.NoError(t, err)
+	assert.True(t, len(CurrentLanguage()) >= 2)
+	assert.Equal(t, "fr", CurrentLanguage()[:2])
+}
+
 func TestSetLanguage_Bad_Unsupported(t *testing.T) {
 	svc, err := New()
 	require.NoError(t, err)
diff --git a/language.go b/language.go
index a93a06f..2ba98b2 100644
--- a/language.go
+++ b/language.go
@@ -2,6 +2,7 @@ package i18n
 
 // GetPluralRule returns the plural rule for a language code.
 func GetPluralRule(lang string) PluralRule {
+	lang = normalizeLanguageTag(lang)
 	if rule, ok := pluralRules[lang]; ok {
 		return rule
 	}
diff --git a/language_test.go b/language_test.go
index 54151fb..fe4537c 100644
--- a/language_test.go
+++ b/language_test.go
@@ -12,11 +12,13 @@ func TestGetPluralCategory(t *testing.T) {
 		{"en", 0, PluralOther},
 		{"en", 1, PluralOne},
 		{"en", 2, PluralOther},
+		{"en_US", 1, PluralOne},
 
 		// French (0 and 1 are singular)
 		{"fr", 0, PluralOne},
 		{"fr", 1, PluralOne},
 		{"fr", 2, PluralOther},
+		{"fr_CA", 2, PluralOther},
 
 		// Russian
 		{"ru", 1, PluralOne},
@@ -88,6 +90,11 @@ func TestGetPluralRule(t *testing.T) {
 		t.Error("Welsh-GB rule(2) should be PluralTwo")
 	}
 
+	rule = GetPluralRule("en_US")
+	if rule(1) != PluralOne {
+		t.Error("English_US rule(1) should be PluralOne")
+	}
+
 	// Unknown falls back to English
 	rule = GetPluralRule("xx-YY")
 	if rule(1) != PluralOne {
diff --git a/localise.go b/localise.go
index e0533e4..d8c485c 100644
--- a/localise.go
+++ b/localise.go
@@ -57,6 +57,7 @@ func (g GrammaticalGender) String() string {
 
 // IsRTLLanguage returns true if the language code uses right-to-left text.
 func IsRTLLanguage(lang string) bool {
+	lang = normalizeLanguageTag(lang)
 	if rtlLanguages[lang] {
 		return true
 	}
@@ -110,8 +111,7 @@ func detectLanguage(supported []language.Tag) string {
 	if langEnv == "" {
 		return ""
 	}
-	baseLang := core.Split(langEnv, ".")[0]
-	baseLang = core.Replace(baseLang, "_", "-")
+	baseLang := normalizeLanguageTag(core.Split(langEnv, ".")[0])
 	parsedLang, err := language.Parse(baseLang)
 	if err != nil {
 		return ""
@@ -129,3 +129,11 @@ func detectLanguage(supported []language.Tag) string {
 	}
 	return ""
 }
+
+func normalizeLanguageTag(lang string) string {
+	lang = core.Trim(lang)
+	if lang == "" {
+		return ""
+	}
+	return core.Replace(lang, "_", "-")
+}
diff --git a/localise_test.go b/localise_test.go
index cefd15c..0ebd191 100644
--- a/localise_test.go
+++ b/localise_test.go
@@ -88,6 +88,7 @@ func TestIsRTLLanguage_Good(t *testing.T) {
 	}{
 		{"arabic", "ar", true},
 		{"arabic_sa", "ar-SA", true},
+		{"arabic_sa_underscore", "ar_EG", true},
 		{"hebrew", "he", true},
 		{"farsi", "fa", true},
 		{"urdu", "ur", true},
diff --git a/service.go b/service.go
index d47edd4..d7d1f78 100644
--- a/service.go
+++ b/service.go
@@ -231,6 +231,7 @@ func (s *Service) loadJSON(lang string, data []byte) error {
 func (s *Service) SetLanguage(lang string) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
+	lang = normalizeLanguageTag(lang)
 	requestedLang, err := language.Parse(lang)
 	if err != nil {
 		return log.E("Service.SetLanguage", "invalid language tag: "+lang, err)
-- 
2.45.3