Don't allow empty "Do Not Translate" list.

Telegram/SourceFiles/boxes/translate_box.cpp

@@ -22,6 +22,7 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 #include "ui/boxes/choose_language_box.h"
 #include "ui/effects/loading_element.h"
 #include "ui/layers/generic_box.h"
+#include "ui/toasts/common_toasts.h"
 #include "ui/painter.h"
 #include "ui/widgets/buttons.h"
 #include "ui/widgets/labels.h"
@@ -39,6 +40,8 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 namespace Ui {
 namespace {
 
+constexpr auto kSkipAtLeastOneDuration = 3 * crl::time(1000);
+
 class ShowButton final : public RpWidget {
 public:
 	ShowButton(not_null<Ui::RpWidget*> parent);
@@ -285,12 +288,36 @@ bool SkipTranslate(TextWithEntities textWithEntities) {
 
 object_ptr<BoxContent> EditSkipTranslationLanguages() {
 	auto title = tr::lng_translate_settings_choose();
-	return Box(ChooseLanguageBox, std::move(title), [=](
+	const auto selected = std::make_shared<std::vector<LanguageId>>(
+		Core::App().settings().skipTranslationLanguages());
+	const auto weak = std::make_shared<QPointer<BoxContent>>();
+	const auto check = [=](LanguageId id) {
+		const auto already = ranges::contains(*selected, id);
+		if (already) {
+			selected->erase(ranges::remove(*selected, id), selected->end());
+		} else {
+			selected->push_back(id);
+		}
+		if (already && selected->empty()) {
+			if (const auto strong = weak->data()) {
+				Ui::ShowMultilineToast({
+					.parentOverride = BoxShow(strong).toastParent(),
+					.text = { tr::lng_translate_settings_one(tr::now) },
+					.duration = kSkipAtLeastOneDuration,
+				});
+			}
+			return false;
+		}
+		return true;
+	};
+	auto result = Box(ChooseLanguageBox, std::move(title), [=](
 			std::vector<LanguageId> &&list) {
 		Core::App().settings().setSkipTranslationLanguages(
 			std::move(list));
 		Core::App().saveSettingsDelayed();
-	}, Core::App().settings().skipTranslationLanguages(), true);
+	}, *selected, true, check);
+	*weak = result.data();
+	return result;
 }
 
 object_ptr<BoxContent> ChooseTranslateToBox(
@@ -310,7 +337,7 @@ object_ptr<BoxContent> ChooseTranslateToBox(
 		Core::App().settings().setTranslateTo(id);
 		Core::App().saveSettingsDelayed();
 		callback(id);
-	}, selected, false);
+	}, selected, false, nullptr);
 }
 
 LanguageId ChooseTranslateTo(not_null<History*> history) {

Telegram/SourceFiles/ui/boxes/choose_language_box.cpp

@@ -266,7 +266,8 @@ void ChooseLanguageBox(
 		rpl::producer<QString> title,
 		Fn<void(std::vector<LanguageId>)> callback,
 		std::vector<LanguageId> selected,
-		bool multiselect) {
+		bool multiselect,
+		Fn<bool(LanguageId)> toggleCheck) {
 	box->setMinHeight(st::boxWidth);
 	box->setMaxHeight(st::boxWidth);
 	box->setTitle(std::move(title));
@@ -294,6 +295,14 @@ void ChooseLanguageBox(
 		});
 		return list;
 	}();
+	struct ToggleOne {
+		LanguageId id;
+		bool selected = false;
+	};
+	struct State {
+		rpl::event_stream<ToggleOne> toggles;
+	};
+	const auto state = box->lifetime().make_state<State>();
 	auto rows = std::vector<not_null<SlideWrap<Row>*>>();
 	rows.reserve(langs.size());
 	for (const auto &id : langs) {
@@ -302,9 +311,21 @@ void ChooseLanguageBox(
 				container,
 				object_ptr<Row>(container, id)));
 		if (multiselect) {
-			button->entity()->toggleOn(
-				rpl::single(ranges::contains(selected, id)),
-				false);
+			button->entity()->toggleOn(rpl::single(
+				ranges::contains(selected, id)
+			) | rpl::then(state->toggles.events(
+			) | rpl::filter([=](ToggleOne one) {
+				return one.id == id;
+			}) | rpl::map([=](ToggleOne one) {
+				return one.selected;
+			})));
+
+			button->entity()->toggledChanges(
+			) | rpl::start_with_next([=](bool value) {
+				if (toggleCheck && !toggleCheck(id)) {
+					state->toggles.fire({ .id = id, .selected = !value });
+				}
+			}, button->lifetime());
 		} else {
 			button->entity()->setClickedCallback([=] {
 				callback({ id });

Telegram/SourceFiles/ui/boxes/choose_language_box.h

@@ -30,6 +30,7 @@ void ChooseLanguageBox(
 	rpl::producer<QString> title,
 	Fn<void(std::vector<LanguageId>)> callback,
 	std::vector<LanguageId> selected,
-	bool multiselect);
+	bool multiselect,
+	Fn<bool(LanguageId)> toggleCheck);
 
 } // namespace Ui