Gui: use parameter observer for locale parameter

Parameter change should be independent of the use of the preference window.

src/Gui/Language/Translator.cpp

@@ -115,6 +115,48 @@ public:
 };
 }
 
+class Translator::ParameterObserver : public ParameterGrp::ObserverType
+{
+public:
+    ParameterObserver(Translator* client) : client(client)
+    {
+        hGrp = App::GetApplication().GetParameterGroupByPath("User parameter:BaseApp/Preferences/General");
+        hGrp->Attach(this);
+    }
+
+    void OnChange(Base::Subject<const char*>& caller, const char* creason) override
+    {
+        (void) caller;
+
+        std::string_view reason = creason;
+        if (reason == "UseLocaleFormatting") {
+            int format = hGrp->GetInt("UseLocaleFormatting");
+            if (format == 0) {
+                client->setLocale(); // Defaults to system locale
+            }
+            else if (format == 1) {
+                // Language must need to be set before locale. How do we ensure this?
+                std::string language = hGrp->GetASCII("Language");
+                client->setLocale(language);
+            }
+            else if (format == 2) {
+                client->setLocale("C");
+            }
+            else {
+                throw Base::ValueError("Parameter \"UseLocaleFormatting\" value out of bounds for Translator::formattingOptions");
+            }
+        }
+        else if (reason == "SubstituteDecimalSeparator") {
+            bool value = hGrp->GetBool("SubstituteDecimal");
+            client->enableDecimalPointConversion(value);
+        }
+    }
+
+    Translator* client;
+    static ParameterGrp::handle hGrp;
+};
+ParameterGrp::handle Translator::ParameterObserver::hGrp;  // definition for export
+
 Translator* Translator::instance()
 {
     if (!_pcSingleton)
@@ -131,6 +173,8 @@ void Translator::destruct ()
 
 Translator::Translator()
 {
+    observer = std::make_unique<Translator::ParameterObserver>(this);
+
     // This is needed for Qt's lupdate
     // clang-format off
     d = new TranslatorP;

src/Gui/Language/Translator.h

@@ -52,6 +52,13 @@ class GuiExport Translator : public QObject
     Q_OBJECT
 
 public:
+    class ParameterObserver;
+    static constexpr std::initializer_list<const char*> formattingOptions{
+        QT_TR_NOOP("Operating system"),
+        QT_TR_NOOP("Selected language"),
+        QT_TR_NOOP("C/POSIX")
+    };
+
     /** @name singleton stuff */
     //@{
     /// Creates an instance
@@ -92,6 +99,7 @@ private:
     void updateLocaleChange() const;
 
 private:
+    std::unique_ptr<ParameterObserver> observer;
     static Translator* _pcSingleton;
     TranslatorP* d;
     std::unique_ptr<Translator, std::function<void(Translator*)>> decimalPointConverter;

src/Gui/PreferencePages/DlgSettingsGeneral.cpp

@@ -85,6 +85,9 @@ DlgSettingsGeneral::DlgSettingsGeneral( QWidget* parent )
 
     recreatePreferencePackMenu();
 
+    for(const char* option : Translator::formattingOptions) {
+        ui->UseLocaleFormatting->addItem(tr(option));
+    }
 
     ui->themesCombobox->setEnabled(true);
     Gui::Document* doc = Gui::Application::Instance->activeDocument();
@@ -172,30 +175,9 @@ void DlgSettingsGeneral::setNumberLocale(bool force/* = false*/)
     if (localeIndex == localeFormat && (!force || localeFormat == 0)) {
         return;
     }
-
-    if (localeFormat == 0) {
-        Translator::instance()->setLocale(); // Defaults to system locale
-    }
-    else if (localeFormat == 1) {
-        QByteArray current = ui->Languages->itemData(ui->Languages->currentIndex()).toByteArray();
-        Translator::instance()->setLocale(current.constData());
-    }
-    else if (localeFormat == 2) {
-        Translator::instance()->setLocale("C");
-    }
-    else {
-        return; // Prevent localeIndex updating if localeFormat is out of range
-    }
     localeIndex = localeFormat;
 }
 
-void DlgSettingsGeneral::setDecimalPointConversion(bool on)
-{
-    if (Translator::instance()->isEnabledDecimalPointConversion() != on) {
-        Translator::instance()->enableDecimalPointConversion(on);
-    }
-}
-
 void DlgSettingsGeneral::saveUnitSystemSettings()
 {
     ParameterGrp::handle hGrpu = App::GetApplication().GetParameterGroupByPath
@@ -256,7 +238,6 @@ void DlgSettingsGeneral::saveSettings()
     bool force = setLanguage();
     // In case type is "Selected language", we need to force locale change
     setNumberLocale(force);
-    setDecimalPointConversion(ui->SubstituteDecimal->isChecked());
 
     ParameterGrp::handle hGrp = WindowParameter::getDefaultParameter()->GetGroup("General");
     QVariant size = ui->toolbarIconSize->itemData(ui->toolbarIconSize->currentIndex());

src/Gui/PreferencePages/DlgSettingsGeneral.ui

@@ -161,21 +161,6 @@
          <property name="prefPath" stdset="0">
           <cstring>General</cstring>
          </property>
-         <item>
-          <property name="text">
-           <string>Operating system</string>
-          </property>
-         </item>
-         <item>
-          <property name="text">
-           <string>Selected language</string>
-          </property>
-         </item>
-         <item>
-          <property name="text">
-           <string>C/POSIX</string>
-          </property>
-         </item>
         </widget>
        </item>
        <item row="4" column="2">