Use QLocale::uiLanguages for system language.

Telegram/SourceFiles/intro/introphone.cpp

@@ -202,8 +202,8 @@ QString PhoneWidget::fullNumber() const {
 	return _code->getLastText() + _phone->getLastText();
 }
 
-void PhoneWidget::selectCountry(const QString &c) {
-	_country->onChooseCountry(c);
+void PhoneWidget::selectCountry(const QString &country) {
+	_country->onChooseCountry(country);
 }
 
 void PhoneWidget::setInnerFocus() {

Telegram/SourceFiles/intro/introwidget.cpp

@@ -43,8 +43,6 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 namespace Intro {
 namespace {
 
-constexpr str_const kDefaultCountry = "US";
-
 void PrepareSupportMode() {
 	using Data::AutoDownload::Full;
 
@@ -70,11 +68,7 @@ Widget::Widget(QWidget *parent, not_null<Main::Account*> account)
 		tr::lng_menu_settings(),
 		st::defaultBoxButton))
 , _next(this, nullptr, st::introNextButton) {
-	auto country = Platform::SystemCountry();
-	if (country.isEmpty()) {
-		country = str_const_toString(kDefaultCountry);
-	}
-	getData()->country = country;
+	getData()->country = Platform::SystemCountry();
 
 	_back->entity()->setClickedCallback([this] { historyMove(Direction::Back); });
 	_back->hide(anim::type::instant);
@@ -439,7 +433,7 @@ void Widget::getNearestDC() {
 			).arg(nearest.vnearest_dc().v
 			).arg(nearest.vthis_dc().v));
 		_account->suggestMainDcId(nearest.vnearest_dc().v);
-		auto nearestCountry = qs(nearest.vcountry());
+		const auto nearestCountry = qs(nearest.vcountry());
 		if (getData()->country != nearestCountry) {
 			getData()->country = nearestCountry;
 			getData()->updated.notify();

Telegram/SourceFiles/platform/linux/info_linux.cpp

@@ -27,13 +27,15 @@ QString SystemVersionPretty() {
 }
 
 QString SystemCountry() {
-	QLocale locale;
-	return locale.name().split('_').last();
+	return QLocale::system().name().split('_').last();
 }
 
 QString SystemLanguage() {
-	QLocale locale;
-	return locale.name().split('_').first();
+	const auto system = QLocale::system();
+	const auto languages = system.uiLanguages();
+	return languages.isEmpty()
+		? system.name().split('_').first()
+		: languages.front();
 }
 
 QDate WhenSystemBecomesOutdated() {

Telegram/SourceFiles/ui/countryinput.cpp

@@ -152,7 +152,9 @@ CountrySelectBox::CountrySelectBox(QWidget*)
 CountrySelectBox::CountrySelectBox(QWidget*, const QString &iso, Type type)
 : _type(type)
 , _select(this, st::contactsMultiSelect, tr::lng_country_ph()) {
-	LastValidISO = iso;
+	if (Data::CountriesByISO2().contains(iso)) {
+		LastValidISO = iso;
+	}
 }
 
 void CountrySelectBox::prepare() {