tools: align locale diagnostics and generate native language names

tools/preview/languages_test.go

@@ -0,0 +1,33 @@
+package preview
+
+import (
+	"path/filepath"
+	"testing"
+
+	"gitea-mail-templates/tools/upstream"
+)
+
+func TestNativeLanguageNames(t *testing.T) {
+	snapshot, err := upstream.LoadPinned(filepath.Join("..", ".."))
+	if err != nil {
+		t.Fatal(err)
+	}
+	locales := snapshot.Languages()
+	names := languageNames(locales)
+	if len(names) != len(locales) {
+		t.Fatal("language names do not match preview locales")
+	}
+	for _, locale := range locales {
+		if names[locale] == "" || names[locale] == locale {
+			t.Errorf("missing native name for %s", locale)
+		}
+	}
+	for locale, want := range map[string]string{"ga-IE": "Gaeilge", "is-IS": "íslenska"} {
+		if names[locale] != want {
+			t.Errorf("%s: got %q, want %q", locale, names[locale], want)
+		}
+	}
+	if got := languageNames([]string{"!invalid"})["!invalid"]; got != "!invalid" {
+		t.Fatalf("invalid locale fallback: %q", got)
+	}
+}