diff --git a/grammar.go b/grammar.go
index 8834a70..38a6609 100644
--- a/grammar.go
+++ b/grammar.go
@@ -12,21 +12,45 @@ import (
 
 // GetGrammarData returns the grammar data for the specified language.
 func GetGrammarData(lang string) *GrammarData {
+	lang = normalizeLanguageTag(lang)
+	if lang == "" {
+		return nil
+	}
 	grammarCacheMu.RLock()
 	defer grammarCacheMu.RUnlock()
-	return grammarCache[lang]
+	if data, ok := grammarCache[lang]; ok && data != nil {
+		return data
+	}
+	if base := baseLanguageTag(lang); base != "" {
+		if data, ok := grammarCache[base]; ok && data != nil {
+			return data
+		}
+	}
+	return nil
 }
 
 // SetGrammarData sets the grammar data for a language, replacing any existing data.
 func SetGrammarData(lang string, data *GrammarData) {
+	lang = normalizeLanguageTag(lang)
+	if lang == "" {
+		return
+	}
 	grammarCacheMu.Lock()
 	defer grammarCacheMu.Unlock()
+	if data == nil {
+		delete(grammarCache, lang)
+		return
+	}
 	grammarCache[lang] = cloneGrammarData(data)
 }
 
 // MergeGrammarData merges grammar data into the existing data for a language.
 // New entries are added; existing entries are overwritten per-key.
 func MergeGrammarData(lang string, data *GrammarData) {
+	lang = normalizeLanguageTag(lang)
+	if lang == "" || data == nil {
+		return
+	}
 	grammarCacheMu.Lock()
 	defer grammarCacheMu.Unlock()
 	existing := grammarCache[lang]
diff --git a/loader_test.go b/loader_test.go
index 8007ebc..eeb949a 100644
--- a/loader_test.go
+++ b/loader_test.go
@@ -454,6 +454,50 @@ func TestMergeGrammarData(t *testing.T) {
 	}
 }
 
+func TestGrammarDataLanguageTagNormalisation(t *testing.T) {
+	const rawLang = "tl_PH"
+	const canonicalLang = "tl-PH"
+
+	originalRaw := GetGrammarData(rawLang)
+	originalCanonical := GetGrammarData(canonicalLang)
+	t.Cleanup(func() {
+		SetGrammarData(rawLang, originalRaw)
+		SetGrammarData(canonicalLang, originalCanonical)
+	})
+
+	SetGrammarData(rawLang, &GrammarData{
+		Words: map[string]string{
+			"demo": "Demo",
+		},
+	})
+
+	if got := GetGrammarData(rawLang); got == nil || got.Words["demo"] != "Demo" {
+		t.Fatalf("GetGrammarData(%q) = %+v, want demo word", rawLang, got)
+	}
+	if got := GetGrammarData(canonicalLang); got == nil || got.Words["demo"] != "Demo" {
+		t.Fatalf("GetGrammarData(%q) = %+v, want demo word", canonicalLang, got)
+	}
+
+	MergeGrammarData(canonicalLang, &GrammarData{
+		Words: map[string]string{
+			"api": "API",
+		},
+	})
+
+	data := GetGrammarData(rawLang)
+	if data == nil {
+		t.Fatalf("GetGrammarData(%q) returned nil after merge", rawLang)
+	}
+	if data.Words["api"] != "API" {
+		t.Fatalf("merged word normalisation failed: %+v", data.Words)
+	}
+
+	SetGrammarData(rawLang, nil)
+	if got := GetGrammarData(canonicalLang); got != nil {
+		t.Fatalf("SetGrammarData(%q, nil) did not clear entry: %+v", rawLang, got)
+	}
+}
+
 func TestNewWithLoader_LoadsGrammarOnlyLocale(t *testing.T) {
 	loaderFS := fstest.MapFS{
 		"fr.json": &fstest.MapFile{