Fix crash due to language handling when opening settings.

Qt doesn't have native language name for some of them. Trying to
capitalize it caused crash.

Use `QLocale(QString)` constructor instead of manually looping and
comparing. The old code incorrectly matched "tr" as "trv".

Don't try to capitalize language name:
* In many cases Qt already returns it capitalized
* capitalization doesn't make sense for some scripts
* in general case splitting first "character" is a hard problem
* in some languages even with latin based scripts name of language isn't
a proper noun which needs to be capitalized

src/common/Configuration.cpp

@@ -244,7 +244,8 @@ bool Configuration::setLocaleByName(const QString &language)
             QLocale::matchingLocales(QLocale::AnyLanguage, QLocale::AnyScript, QLocale::AnyCountry);
 
     for (auto &it : allLocales) {
-        if (QString::compare(it.nativeLanguageName(), language, Qt::CaseInsensitive) == 0) {
+        if (QString::compare(it.nativeLanguageName(), language, Qt::CaseInsensitive) == 0 ||
+            it.name() == language) {
             setLocale(it);
             return true;
         }
@@ -650,14 +651,16 @@ QStringList Configuration::getAvailableTranslations()
             QLocale::matchingLocales(QLocale::AnyLanguage, QLocale::AnyScript, QLocale::AnyCountry);
 
     for (auto i : fileNames) {
-        QString localeName = i.mid(sizeof("cutter_") - 1, 2);
-        for (auto j : allLocales) {
-            if (j.name().startsWith(localeName)) {
-                currLanguageName = j.nativeLanguageName();
-                currLanguageName = currLanguageName.at(0).toUpper()
-                        + currLanguageName.right(currLanguageName.length() - 1);
+        QString localeName = i.mid(sizeof("cutter_") - 1, 2); // TODO:#2321 don't asume 2 characters
+        // language code is sometimes 3 characters, and there could also be language_COUNTRY. Qt supports that.
+        QLocale locale(localeName);
+        if (locale.language() != QLocale::C) {
+            currLanguageName = locale.nativeLanguageName();
+            if (currLanguageName.isEmpty()) { // Qt doesn't have native language name for some languages
+                currLanguageName = QLocale::languageToString(locale.language());
+            }
+            if (!currLanguageName.isEmpty()) {
                 languages << currLanguageName;
-                break;
             }
         }
     }

src/dialogs/WelcomeDialog.cpp

@@ -26,7 +26,6 @@ WelcomeDialog::WelcomeDialog(QWidget *parent) : QDialog(parent), ui(new Ui::Welc
     QStringList langs = Config()->getAvailableTranslations();
     ui->languageComboBox->addItems(langs);
     QString curr = Config()->getCurrLocale().nativeLanguageName();
-    curr = curr.at(0).toUpper() + curr.right(curr.length() - 1);
     if (!langs.contains(curr)) {
         curr = "English";
     }
@@ -64,7 +63,7 @@ void WelcomeDialog::on_themeComboBox_currentIndexChanged(int index)
  */
 void WelcomeDialog::onLanguageComboBox_currentIndexChanged(int index)
 {
-    QString language = ui->languageComboBox->itemText(index).toLower();
+    QString language = ui->languageComboBox->itemText(index);
     Config()->setLocaleByName(language);
 
     QMessageBox mb;

src/dialogs/preferences/AppearanceOptionsWidget.cpp

@@ -33,7 +33,6 @@ AppearanceOptionsWidget::AppearanceOptionsWidget(PreferencesDialog *dialog)
     ui->languageComboBox->addItems(langs);
 
     QString curr = Config()->getCurrLocale().nativeLanguageName();
-    curr = curr.at(0).toUpper() + curr.right(curr.length() - 1);
     if (!langs.contains(curr)) {
         curr = "English";
     }