FEATURE: Setting preferred locale and fallback manually

lang/lang.go

@@ -16,6 +16,7 @@
 	"fyne.io/fyne/v2"
 
 	"golang.org/x/text/language"
+	"slices"
 )
 
 var (
@@ -35,6 +36,12 @@
 	// More info available on the `LocalizePluralKey` function.
 	XN = LocalizePluralKey
 
+	// This defines an order in which it will try to find a fallback in case localizer does not find a match.
+	// All other languages will be in order as the system reads them (which is most likely alphabetical).
+	languageOrder = []string{"en"}
+
+	preferredLanguage string
+
 	bundle    *i18n.Bundle
 	localizer *i18n.Localizer
 
@@ -159,6 +166,22 @@
 	return retErr
 }
 
+// SetLanguageOrder allows an app to set the order in which translations are checked in case no locale matches.
+// Since 2.6
+func SetLanguageOrder(order []string) {
+	languageOrder = order
+	updateLocalizer()
+}
+
+
+// SetPreferredLocale allows an app to set the preferred locale for translations, overwriting the System Locale.
+// locale can be in format en_US_someVariant, en_US, en-US-someVariant, en-US, en
+// Since 2.6
+func SetPreferredLocale(locale string) {
+	preferredLanguage = locale
+	updateLocalizer()
+}
+
 func addLanguage(data []byte, name string) error {
 	f, err := bundle.ParseMessageFileBytes(data, name)
 	translated = append(translated, f.Tag)
@@ -186,18 +209,52 @@
 	return str.String()
 }
 
+func orderLanguages(a, b language.Tag) int {
+	indexA := -1
+	indexB := -1
+	for i, l := range languageOrder {
+		if a.String() == l {
+			indexA = i
+		}
+		if b.String() == l {
+			indexB = i
+		}
+	}
+	// Order both languages as defined in languageOrder
+	if indexA != -1 && indexB != -1 {
+		return indexA - indexB
+	}
+	// If it is the only language in languageOrder, it comes first
+	if indexA != -1 {
+		return -1
+	}
+	if indexB != -1 {
+		return 1
+	}
+	// If no language is in languageOrder, sort alphabetically
+	return strings.Compare(a.String(), b.String())
+}
+
 // A utility for setting up languages - available to unit tests for overriding system
 func setupLang(lang string) {
 	localizer = i18n.NewLocalizer(bundle, lang)
 }
 
 // updateLocalizer Finds the closest translation from the user's locale list and sets it up
 func updateLocalizer() {
+	// Sort the translated slice using the orderLanguages function
+	slices.SortStableFunc(translated, func(a, b language.Tag) int {
+		return orderLanguages(a, b)
+	})
+
 	all, err := locale.GetLocales()
 	if err != nil {
 		fyne.LogError("Failed to load user locales", err)
 		all = []string{"en"}
 	}
+	if preferredLanguage != "" {
+		all = []string{preferredLanguage}
+	}
 	str := closestSupportedLocale(all).LanguageString()
 	setupLang(str)
 	localizer = i18n.NewLocalizer(bundle, str)

lang/lang_test.go

@@ -76,3 +76,32 @@
 	assert.Equal(t, "Apple", XN("appleID", "Apple", 1))
 	assert.Equal(t, "Apples", XN("appleID", "Apple", 2))
 }
+
+func TestSetPreferredLocale(t *testing.T) {
+	_ = AddTranslations(fyne.NewStaticResource("en.json", []byte(`{
+  "Test": "Match"
+}`)))
+	_ = AddTranslations(fyne.NewStaticResource("fr.json", []byte(`{
+  "Test2": "Match2"
+}`)))
+	setupLang("fr")
+	assert.Equal(t, "Match2", L("Test2"))
+	SetPreferredLocale("en")
+	assert.Equal(t, "Match", L("Test"))
+}
+
+func TestSetLanguageOrder(t *testing.T) {
+	_ = AddTranslations(fyne.NewStaticResource("en.json", []byte(`{
+  "Test": "Match"
+}`)))
+	_ = AddTranslations(fyne.NewStaticResource("fr.json", []byte(`{
+  "Test2": "Match2"
+}`)))
+	setupLang("xyz") // invalid language to test fallback
+	SetLanguageOrder("en")
+	assert.Equal(t, "Match", L("Test"))
+	SetLanguageOrder("fr")
+	assert.Equal(t, "Match2", L("Test2"))
+	setupLang("en")
+	assert.Equal(t, "Match", L("Test"))
+}