From 277445cc5d9f25816d5a10fa22919faab91e98e8 Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Wed, 1 Apr 2026 23:06:02 +0000
Subject: [PATCH] fix(service): re-detect registered locales after init

Co-Authored-By: Virgil <virgil@lethean.io>
---
 hooks_test.go | 37 +++++++++++++++++++++++++++++++++++
 localise.go   |  5 ++++-
 service.go    | 54 ++++++++++++++++++++++++++++++++++++++-------------
 3 files changed, 82 insertions(+), 14 deletions(-)

diff --git a/hooks_test.go b/hooks_test.go
index 7727fb6..52f0e6b 100644
--- a/hooks_test.go
+++ b/hooks_test.go
@@ -110,6 +110,43 @@ func TestInit_LoadsRegisteredLocales(t *testing.T) {
 	assert.Equal(t, "loaded on init", got)
 }
 
+func TestInit_ReDetectsRegisteredLocales(t *testing.T) {
+	t.Setenv("LANG", "de_DE.UTF-8")
+
+	registeredLocalesMu.Lock()
+	savedLocales := registeredLocales
+	savedLoaded := localesLoaded
+	registeredLocales = nil
+	localesLoaded = false
+	registeredLocalesMu.Unlock()
+
+	defaultOnce = sync.Once{}
+	defaultService.Store(nil)
+
+	defer func() {
+		registeredLocalesMu.Lock()
+		registeredLocales = savedLocales
+		localesLoaded = savedLoaded
+		registeredLocalesMu.Unlock()
+		defaultService.Store(nil)
+		defaultOnce = sync.Once{}
+	}()
+
+	fs := fstest.MapFS{
+		"locales/de.json": &fstest.MapFile{
+			Data: []byte(`{"hello": "hallo"}`),
+		},
+	}
+	RegisterLocales(fs, "locales")
+
+	require.NoError(t, Init())
+
+	svc := Default()
+	require.NotNil(t, svc)
+	assert.Contains(t, svc.Language(), "de")
+	assert.Equal(t, "hallo", svc.T("hello"))
+}
+
 func TestLoadRegisteredLocales_Good(t *testing.T) {
 	svc, err := New()
 	require.NoError(t, err)
diff --git a/localise.go b/localise.go
index 794368c..b4a833b 100644
--- a/localise.go
+++ b/localise.go
@@ -105,8 +105,11 @@ func detectLanguage(supported []language.Tag) string {
 		return ""
 	}
 	matcher := language.NewMatcher(supported)
-	bestMatch, _, confidence := matcher.Match(parsedLang)
+	bestMatch, bestIndex, confidence := matcher.Match(parsedLang)
 	if confidence >= language.Low {
+		if bestIndex >= 0 && bestIndex < len(supported) {
+			return supported[bestIndex].String()
+		}
 		return bestMatch.String()
 	}
 	return ""
diff --git a/service.go b/service.go
index 20a33bd..fb4b8df 100644
--- a/service.go
+++ b/service.go
@@ -16,16 +16,17 @@ import (
 
 // Service provides grammar-aware internationalisation.
 type Service struct {
-	loader         Loader
-	messages       map[string]map[string]Message // lang -> key -> message
-	currentLang    string
-	fallbackLang   string
-	availableLangs []language.Tag
-	mode           Mode
-	debug          bool
-	formality      Formality
-	handlers       []KeyHandler
-	mu             sync.RWMutex
+	loader           Loader
+	messages         map[string]map[string]Message // lang -> key -> message
+	currentLang      string
+	fallbackLang     string
+	languageExplicit bool
+	availableLangs   []language.Tag
+	mode             Mode
+	debug            bool
+	formality        Formality
+	handlers         []KeyHandler
+	mu               sync.RWMutex
 }
 
 // Option configures a Service during construction.
@@ -222,11 +223,16 @@ func (s *Service) SetLanguage(lang string) error {
 		return log.E("Service.SetLanguage", "no languages available", nil)
 	}
 	matcher := language.NewMatcher(s.availableLangs)
-	bestMatch, _, confidence := matcher.Match(requestedLang)
+	bestMatch, bestIndex, confidence := matcher.Match(requestedLang)
 	if confidence == language.No {
 		return log.E("Service.SetLanguage", "unsupported language: "+lang, nil)
 	}
-	s.currentLang = bestMatch.String()
+	if bestIndex >= 0 && bestIndex < len(s.availableLangs) {
+		s.currentLang = s.availableLangs[bestIndex].String()
+	} else {
+		s.currentLang = bestMatch.String()
+	}
+	s.languageExplicit = true
 	return nil
 }
 
@@ -571,6 +577,7 @@ func (s *Service) AddLoader(loader Loader) error {
 		}
 		s.mu.Unlock()
 	}
+	s.autoDetectLanguage()
 	return nil
 }
 
@@ -578,7 +585,17 @@ func (s *Service) AddLoader(loader Loader) error {
 // Deprecated: Use AddLoader(NewFSLoader(fsys, dir)) instead for proper grammar handling.
 func (s *Service) LoadFS(fsys fs.FS, dir string) error {
 	s.mu.Lock()
-	defer s.mu.Unlock()
+	defer func() {
+		s.mu.Unlock()
+		if s.languageExplicit {
+			return
+		}
+		if detected := detectLanguage(s.availableLangs); detected != "" {
+			s.mu.Lock()
+			s.currentLang = detected
+			s.mu.Unlock()
+		}
+	}()
 	entries, err := fs.ReadDir(fsys, dir)
 	if err != nil {
 		return log.E("Service.LoadFS", "read locales directory", err)
@@ -605,3 +622,14 @@ func (s *Service) LoadFS(fsys fs.FS, dir string) error {
 	}
 	return nil
 }
+
+func (s *Service) autoDetectLanguage() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.languageExplicit {
+		return
+	}
+	if detected := detectLanguage(s.availableLangs); detected != "" {
+		s.currentLang = detected
+	}
+}
-- 
2.45.3