diff --git a/context.go b/context.go
index 0ef7d71..6d9b9f2 100644
--- a/context.go
+++ b/context.go
@@ -31,17 +31,35 @@ func applyLocaleToService(svc Translator, locale string) {
 	}
 
 	if setter, ok := svc.(interface{ SetLanguage(string) error }); ok {
-		base := locale
-		for i := 0; i < len(base); i++ {
-			if base[i] == '-' || base[i] == '_' {
-				base = base[:i]
-				break
-			}
+		// The built-in i18n service matches best on a base language tag.
+		// Custom translators should receive the full locale string.
+		if serviceUsesBaseLanguage(svc) {
+			locale = baseLanguage(locale)
 		}
-		_ = setter.SetLanguage(base)
+		_ = setter.SetLanguage(locale)
 	}
 }
 
+func serviceUsesBaseLanguage(svc Translator) bool {
+	t := reflect.TypeOf(svc)
+	for t != nil && t.Kind() == reflect.Pointer {
+		t = t.Elem()
+	}
+	if t == nil {
+		return false
+	}
+	return t.PkgPath() == "dappco.re/go/core/i18n" && t.Name() == "Service"
+}
+
+func baseLanguage(locale string) string {
+	for i := 0; i < len(locale); i++ {
+		if locale[i] == '-' || locale[i] == '_' {
+			return locale[:i]
+		}
+	}
+	return locale
+}
+
 // NewContext creates a new rendering context with sensible defaults.
 // Usage example: html := Render(Text("welcome"), NewContext("en-GB"))
 func NewContext(locale ...string) *Context {
diff --git a/context_test.go b/context_test.go
index a58a42e..924b9d8 100644
--- a/context_test.go
+++ b/context_test.go
@@ -10,10 +10,16 @@ import (
 )
 
 type recordingTranslator struct {
+	lang string
 	key  string
 	args []any
 }
 
+func (r *recordingTranslator) SetLanguage(lang string) error {
+	r.lang = lang
+	return nil
+}
+
 func (r *recordingTranslator) T(key string, args ...any) string {
 	r.key = key
 	r.args = append(r.args[:0], args...)
@@ -59,6 +65,18 @@ func TestNewContextWithService_AppliesLocaleToService_Good(t *testing.T) {
 	}
 }
 
+func TestNewContextWithService_ForwardsFullLocaleToTranslator_Good(t *testing.T) {
+	svc := &recordingTranslator{}
+	ctx := NewContextWithService(svc, "en-GB")
+
+	if ctx == nil {
+		t.Fatal("NewContextWithService returned nil")
+	}
+	if svc.lang != "en-GB" {
+		t.Fatalf("NewContextWithService forwarded locale = %q, want %q", svc.lang, "en-GB")
+	}
+}
+
 func TestTextNode_UsesMetadataAliasWhenDataNil_Good(t *testing.T) {
 	svc, _ := i18n.New()
 	i18n.SetDefault(svc)
@@ -143,6 +161,19 @@ func TestContext_SetLocale_AppliesLocale_Good(t *testing.T) {
 	}
 }
 
+func TestContext_SetLocale_ForwardsFullLocaleToTranslator_Good(t *testing.T) {
+	ctx := NewContextWithService(&recordingTranslator{})
+
+	if got := ctx.SetLocale("fr-FR"); got != ctx {
+		t.Fatal("SetLocale should return the same context for chaining")
+	}
+
+	svc := ctx.service.(*recordingTranslator)
+	if svc.lang != "fr-FR" {
+		t.Fatalf("SetLocale forwarded locale = %q, want %q", svc.lang, "fr-FR")
+	}
+}
+
 func TestContext_SetLocale_NilContext_Ugly(t *testing.T) {
 	var ctx *Context
 	if got := ctx.SetLocale("en-GB"); got != nil {