feat(i18n): add base language grammar fallback

Co-Authored-By: Virgil <virgil@lethean.io>

grammar.go

@@ -190,7 +190,7 @@ func Upper(s string) string {
 }
 
 func getVerbForm(lang, verb, form string) string {
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data == nil || data.Verbs == nil {
 		return ""
 	}
@@ -207,7 +207,7 @@ func getVerbForm(lang, verb, form string) string {
 }
 
 func getWord(lang, word string) string {
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data == nil || data.Words == nil {
 		return ""
 	}
@@ -215,7 +215,7 @@ func getWord(lang, word string) string {
 }
 
 func getPunct(lang, rule, defaultVal string) string {
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data == nil {
 		return defaultVal
 	}
@@ -233,7 +233,7 @@ func getPunct(lang, rule, defaultVal string) string {
 }
 
 func getNounForm(lang, noun, form string) string {
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data == nil || data.Nouns == nil {
 		return ""
 	}
@@ -477,7 +477,7 @@ func Article(word string) string {
 
 func articleForCurrentLanguage(lowerWord, originalWord string) (string, bool) {
 	lang := currentLangForGrammar()
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data == nil {
 		return "", false
 	}
@@ -768,7 +768,7 @@ func DefiniteArticle(word string) string {
 		return article
 	}
 	lang := currentLangForGrammar()
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data != nil && data.Articles.Definite != "" {
 		return data.Articles.Definite
 	}
@@ -794,7 +794,7 @@ func DefinitePhrase(word string) string {
 
 func definiteArticleForCurrentLanguage(lowerWord, originalWord string) (string, bool) {
 	lang := currentLangForGrammar()
-	data := GetGrammarData(lang)
+	data := grammarDataForLang(lang)
 	if data == nil {
 		return "", false
 	}
@@ -807,6 +807,23 @@ func definiteArticleForCurrentLanguage(lowerWord, originalWord string) (string,
 	return "", false
 }
 
+func grammarDataForLang(lang string) *GrammarData {
+	if data := GetGrammarData(lang); data != nil {
+		return data
+	}
+	if base := baseLanguageTag(lang); base != "" {
+		return GetGrammarData(base)
+	}
+	return nil
+}
+
+func baseLanguageTag(lang string) string {
+	if idx := indexAny(lang, "-_"); idx > 0 {
+		return lang[:idx]
+	}
+	return ""
+}
+
 func definiteArticleFromGrammarForms(data *GrammarData, lowerWord, originalWord, lang string) (string, bool) {
 	if data == nil || data.Articles.Definite == "" {
 		return "", false

grammar_test.go

@@ -7,6 +7,16 @@ import (
 	"time"
 )
 
+type regionFallbackLoader struct{}
+
+func (regionFallbackLoader) Languages() []string {
+	return []string{"en-GB"}
+}
+
+func (regionFallbackLoader) Load(lang string) (map[string]Message, *GrammarData, error) {
+	return map[string]Message{}, nil, nil
+}
+
 func TestPastTense(t *testing.T) {
 	// Ensure grammar data is loaded from embedded JSON
 	svc, err := New()
@@ -933,6 +943,58 @@ func TestFrenchGrammarData(t *testing.T) {
 	}
 }
 
+func TestGrammarFallbackToBaseLanguageTag(t *testing.T) {
+	prevDefault := Default()
+	prevGrammar := GetGrammarData("en")
+	t.Cleanup(func() {
+		SetGrammarData("en", prevGrammar)
+		SetDefault(prevDefault)
+	})
+
+	SetGrammarData("en", &GrammarData{
+		Verbs: map[string]VerbForms{
+			"delete": {Past: "deleted", Gerund: "deleting"},
+		},
+		Nouns: map[string]NounForms{
+			"file": {One: "file", Other: "files"},
+		},
+		Articles: ArticleForms{
+			IndefiniteDefault: "a",
+			IndefiniteVowel:   "an",
+			Definite:          "the",
+		},
+		Punct: PunctuationRules{
+			LabelSuffix:    ":",
+			ProgressSuffix: "...",
+		},
+		Words: map[string]string{
+			"status": "Status",
+		},
+	})
+
+	svc, err := NewWithLoader(regionFallbackLoader{})
+	if err != nil {
+		t.Fatalf("NewWithLoader() failed: %v", err)
+	}
+	SetDefault(svc)
+	if err := svc.SetLanguage("en-GB"); err != nil {
+		t.Fatalf("SetLanguage(en-GB) failed: %v", err)
+	}
+
+	if got := PastTense("delete"); got != "deleted" {
+		t.Fatalf("PastTense(delete) = %q, want %q", got, "deleted")
+	}
+	if got := Pluralize("file", 2); got != "files" {
+		t.Fatalf("Pluralize(file, 2) = %q, want %q", got, "files")
+	}
+	if got := Article("apple"); got != "an" {
+		t.Fatalf("Article(apple) = %q, want %q", got, "an")
+	}
+	if got := Label("status"); got != "Status:" {
+		t.Fatalf("Label(status) = %q, want %q", got, "Status:")
+	}
+}
+
 func TestTemplateFuncs(t *testing.T) {
 	funcs := TemplateFuncs()
 	expected := []string{