diff --git a/server/internal/adapter/context.go b/server/internal/adapter/context.go
index 5477bba05..cf0851cae 100644
--- a/server/internal/adapter/context.go
+++ b/server/internal/adapter/context.go
@@ -34,7 +34,7 @@ type AuthInfo struct {
 	EmailVerified *bool
 }
 
-func AttachLang(ctx context.Context, lang string) context.Context {
+func AttachLang(ctx context.Context, lang language.Tag) context.Context {
 	return context.WithValue(ctx, contextLang, lang)
 }
 
@@ -74,11 +74,11 @@ func Lang(ctx context.Context, lang *language.Tag) string {
 	}
 
 	if v := ctx.Value(contextLang); v != nil {
-		if lang, ok := v.(string); ok {
-			if lang == "" {
+		if lang, ok := v.(language.Tag); ok {
+			if lang.IsRoot() {
 				return defaultLang.String()
 			}
-			return lang
+			return lang.String()
 		}
 	}
 
diff --git a/server/internal/adapter/context_test.go b/server/internal/adapter/context_test.go
new file mode 100644
index 000000000..241d5b67c
--- /dev/null
+++ b/server/internal/adapter/context_test.go
@@ -0,0 +1,54 @@
+package adapter
+
+import (
+	"context"
+	"testing"
+
+	"github.com/reearth/reearthx/log"
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/text/language"
+)
+
+func TestLang(t *testing.T) {
+
+	// Default language for testing
+	defaultLang := language.English // or set it to whatever your defaultLang is
+
+	t.Run("Lang is provided and valid", func(t *testing.T) {
+		lang := language.Japanese
+		result := Lang(context.Background(), &lang)
+		assert.Equal(t, "ja", result)
+	})
+
+	t.Run("Lang is nil, context has valid lang", func(t *testing.T) {
+		lang := language.French
+		ctx := context.WithValue(context.Background(), contextLang, lang)
+		result := Lang(ctx, nil)
+		log.Infofc(ctx, "result: %s", result)
+		assert.Equal(t, "fr", result)
+	})
+
+	t.Run("Lang is nil, context lang is empty", func(t *testing.T) {
+		ctx := context.WithValue(context.Background(), contextLang, language.Make(""))
+		result := Lang(ctx, nil)
+		assert.Equal(t, defaultLang.String(), result)
+	})
+
+	t.Run("Lang is nil, context has no lang", func(t *testing.T) {
+		result := Lang(context.Background(), nil)
+		assert.Equal(t, defaultLang.String(), result)
+	})
+
+	t.Run("Lang is root, context has no lang", func(t *testing.T) {
+		rootLang := language.Und
+		result := Lang(context.Background(), &rootLang)
+		assert.Equal(t, defaultLang.String(), result)
+	})
+
+	t.Run("Lang is french, context has no lang", func(t *testing.T) {
+		lang := language.French
+		result := Lang(context.Background(), &lang)
+		assert.Equal(t, lang.String(), result)
+	})
+
+}
diff --git a/server/internal/app/graphql_test.go b/server/internal/app/graphql_test.go
index 3f32eeb32..a4cb3211b 100644
--- a/server/internal/app/graphql_test.go
+++ b/server/internal/app/graphql_test.go
@@ -9,11 +9,12 @@ import (
 	"github.com/reearth/reearth/server/pkg/apperror"
 	"github.com/stretchr/testify/assert"
 	"github.com/vektah/gqlparser/v2/ast"
+	"golang.org/x/text/language"
 )
 
 func TestCustomErrorPresenter(t *testing.T) {
 	ctx := context.Background()
-	ctx = adapter.AttachLang(ctx, "en")
+	ctx = adapter.AttachLang(ctx, language.English)
 
 	appErr := &apperror.AppError{
 		LocalesError: map[string]*apperror.LocalesError{
diff --git a/server/internal/app/lang.go b/server/internal/app/lang.go
index 7d19063ef..1d04062b0 100644
--- a/server/internal/app/lang.go
+++ b/server/internal/app/lang.go
@@ -5,10 +5,11 @@ import (
 
 	"github.com/labstack/echo/v4"
 	"github.com/reearth/reearth/server/internal/adapter"
+	"golang.org/x/text/language"
 )
 
 // LanguageExtractor extracts the appropriate language from the request.
-func LanguageExtractor(req *http.Request) string {
+func LanguageExtractor(req *http.Request) language.Tag {
 	// Extract browser language from header
 	lang := req.Header.Get("lang")
 
@@ -20,7 +21,12 @@ func LanguageExtractor(req *http.Request) string {
 		lang = u.Lang().String()
 	}
 
-	return lang
+	tag, err := language.Parse(lang)
+	if err != nil {
+		return language.English
+	}
+
+	return tag
 }
 
 // AttachLanguageMiddleware attaches the detected language to the request context.
diff --git a/server/internal/app/lang_test.go b/server/internal/app/lang_test.go
index 322c45f16..fe4b0d8b6 100644
--- a/server/internal/app/lang_test.go
+++ b/server/internal/app/lang_test.go
@@ -21,25 +21,25 @@ func TestLanguageExtractor(t *testing.T) {
 		name       string
 		headerLang string
 		userLang   language.Tag
-		expected   string
+		expected   language.Tag
 	}{
 		{
 			name:       "User language overrides browser language",
 			headerLang: "fr",
 			userLang:   language.English,
-			expected:   "en",
+			expected:   language.English,
 		},
 		{
 			name:       "No user language, use browser language",
 			headerLang: "fr",
 			userLang:   language.Und,
-			expected:   "fr",
+			expected:   language.French,
 		},
 		{
 			name:       "No browser language or user language is und, fallback to default",
 			headerLang: "",
 			userLang:   language.Und,
-			expected:   "",
+			expected:   language.English,
 		},
 	}