Fall back to English if there is no sensible match

Fixes fyne-io#5040

lang/lang.go

@@ -169,6 +169,7 @@ func init() {
 	bundle = i18n.NewBundle(language.English)
 	bundle.RegisterUnmarshalFunc("json", json.Unmarshal)
 
+	translated = []language.Tag{language.Make("en")} // the first item in this list will be the fallback if none match
 	err := AddTranslationsFS(translations, "translations")
 	if err != nil {
 		fyne.LogError("Error occurred loading built-in translations", err)

lang/lang_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"fyne.io/fyne/v2"
+	"github.com/nicksnyder/go-i18n/v2/i18n"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -23,6 +24,29 @@ func TestAddTranslations(t *testing.T) {
 	assert.Equal(t, "Match2", L("Test2"))
 }
 
+func TestLocalize_Default(t *testing.T) {
+	setupLang("en")
+	err := AddTranslationsForLocale([]byte(`{
+  "Test": "Match"
+}`), "en")
+	assert.Nil(t, err)
+
+	err = AddTranslationsForLocale([]byte(`{
+  "Test": "Match2"
+}`), "de")
+	assert.Nil(t, err)
+
+	fr := i18n.NewLocalizer(bundle, "fr")
+	ret, err := fr.Localize(&i18n.LocalizeConfig{
+		DefaultMessage: &i18n.Message{
+			ID: "Test",
+		},
+	})
+
+	assert.NotNil(t, err) // we are falling back
+	assert.Equal(t, "Match", ret)
+}
+
 func TestLocalize_Fallback(t *testing.T) {
 	assert.Equal(t, "Missing", L("Missing"))
 }