feat: allow overriding the language through the configuration

e.g. this config entry:

```ini
; nextcloud.cfg
[General]
language=de
```

will ensure that the application will always be translated to German.

It's also possible to set the configuration option from the command
line:

```sh
# set the language to German for the next client startup
./nextcloud --set-language de

# set the language to the OS default for the next client startup
./nextcloud --set-language ""
```

partial fix for #111, as this is not a GUI option

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>

# Date:      Mon Mar 31 12:44:55 2025 +0200
#
# On branch feature/enforce-translations
# Your branch and 'origin/feature/enforce-translations' have diverged,
# and have 1 and 1 different commits each, respectively.
#
# Changes to be committed:
#	modified:   src/gui/application.cpp
#	modified:   src/gui/application.h
#	modified:   src/libsync/configfile.cpp
#	modified:   src/libsync/configfile.h
#
# Untracked files:
#	.kateproject.build
#	metainfo.po
#	test.metainfo.xml
#

Author: nilsding

src/gui/application.cpp

@@ -97,6 +97,7 @@ namespace {
         "  --overridelocaldir         : specify a local dir to be used in the account setup wizard.\n"
         "  --userid                   : userId (username as on the server) to pass when creating an account via command-line.\n"
         "  --apppassword              : appPassword to pass when creating an account via command-line.\n"
+        "  --set-language <lang>      : specify a language to use for the client, regardless of the OS language.\n"
         "  --localdirpath             : (optional) path where to create a local sync folder when creating an account via command-line.\n"
         "  --isvfsenabled             : whether to set a VFS or non-VFS folder (1 for 'yes' or 0 for 'no') when creating an account via command-line.\n"
         "  --remotedirpath            : (optional) path to a remote subfolder when creating an account via command-line.\n"
@@ -362,6 +363,12 @@ Application::Application(int &argc, char **argv)
             shouldExit = true;
         }
 
+        if (!_setLanguage.isNull()) {
+            // checking for .isNull here as setting the value to an empty string will use the OS language again
+            cfg.setLanguage(_setLanguage);
+            shouldExit = true;
+        }
+
         if (AccountSetupCommandLineManager::instance()) {
             cfg.setVfsEnabled(AccountSetupCommandLineManager::instance()->isVfsEnabled());
         }
@@ -881,6 +888,10 @@ void Application::parseOptions(const QStringList &options)
             } else {
                 showHint("Invalid URL passed to --overridelocaldir");
             }
+        } else if (option == QStringLiteral("--set-language")) {
+            if (it.hasNext() && !it.peekNext().startsWith(QLatin1String("--"))) {
+                _setLanguage = it.next();
+            }
         } else if (option == QStringLiteral("--forcelegacyconfigimport")) {
             AccountManager::instance()->setForceLegacyImport(true);
         } else {
@@ -1001,10 +1012,22 @@ QString substLang(const QString &lang)
     return lang;
 }
 
+QString enforcedLanguage()
+{
+    const ConfigFile cfg;
+    const auto configLanguage = cfg.language();
+    if (!configLanguage.isEmpty()) {
+        // always prefer value from configuration
+        return configLanguage;
+    }
+
+    return Theme::instance()->enforcedLocale();
+}
+
 void Application::setupTranslations()
 {
     qCInfo(lcApplication) << "System UI languages are:" << QLocale::system().uiLanguages();
-    const auto enforcedLocale = Theme::instance()->enforcedLocale();
+    const auto enforcedLocale = enforcedLanguage();
     const auto lang = substLang(!enforcedLocale.isEmpty() ? enforcedLocale : QLocale::system().uiLanguages(QLocale::TagSeparator::Underscore).first());
     qCInfo(lcApplication) << "selected application language:" << lang;
 

src/gui/application.h

@@ -150,6 +150,7 @@ protected slots:
 
     QString _overrideServerUrl;
     QString _overrideLocalDir;
+    QString _setLanguage;
 
 #if defined(WITH_CRASHREPORTER)
     QScopedPointer<CrashReporter::Handler> _crashHandler;

src/libsync/configfile.cpp

@@ -115,6 +115,8 @@ static const QString defaultUpdateChannelName = "stable";
 static const QStringList enterpriseUpdateChannelsList { QStringLiteral("stable"), QStringLiteral("enterprise") };
 static const QString defaultEnterpriseChannel = "enterprise";
 
+static constexpr char languageC[] = "language";
+
 static constexpr int deleteFilesThresholdDefaultValue = 100;
 }
 
@@ -1252,6 +1254,17 @@ void ConfigFile::setDesktopEnterpriseChannel(const QString &channel)
     settings.setValue(QLatin1String(desktopEnterpriseChannelName), channel);
 }
 
+QString ConfigFile::language() const
+{
+    QSettings settings(configFile(), QSettings::IniFormat);
+    return settings.value(QLatin1String(languageC), QLatin1String("")).toString();
+}
+
+void ConfigFile::setLanguage(const QString& language)
+{
+    QSettings settings(configFile(), QSettings::IniFormat);
+    settings.setValue(QLatin1String(languageC), language);
+}
 
 Q_GLOBAL_STATIC(QString, g_configFileName)
 

src/libsync/configfile.h

@@ -241,6 +241,10 @@ class OWNCLOUDSYNC_EXPORT ConfigFile
     [[nodiscard]] QString desktopEnterpriseChannel() const;
     void setDesktopEnterpriseChannel(const QString &channel);
 
+    /// Enforce a specific language used for the UI
+    [[nodiscard]] QString language() const;
+    void setLanguage(const QString &language);
+
     /**  Returns a new settings pre-set in a specific group.  The Settings will be created
          with the given parent. If no parent is specified, the caller must destroy the settings */
     static std::unique_ptr<QSettings> settingsWithGroup(const QString &group, QObject *parent = nullptr);