i18n: English/German from Switch system language

- Add I18n module: German when system language is DE, else English (ENUS/ENGB
  or any other language).
- Translate UI strings (tabs, settings, about, file browser, notifications,
  OC row titles, toggles Ein/Aus).
- Keep toggle/frequency/voltage list rows without subtitle descriptions
  like before.

Uses setGetSystemLanguage + setMakeLanguage; call I18n::init before
Application::init.

Made-with: Cursor

src/i18n.h

@@ -0,0 +1,57 @@
+/*
+    SWR INI Tool - Switchroot INI Configuration Editor
+    Copyright (C) 2026 Switchroot
+*/
+
+#pragma once
+
+#include <string>
+
+namespace I18n
+{
+
+void init();
+
+bool isGerman();
+
+const char* appTitle();
+
+const char* tabSettings();
+const char* tabAbout();
+
+const char* wordOn();
+const char* wordOff();
+
+std::string sectionEditLine(const std::string& section, const std::string& path);
+
+const char* headerToggleOptions();
+const char* headerFrequencySettings();
+const char* headerVoltageSettings();
+const char* headerOther();
+const char* headerIniFilePaths();
+const char* headerLinksAndResources();
+
+const char* errorListItemTitleIni();
+std::string iniLoadErrorBody(const std::string& path);
+std::string iniSectionErrorBody(const std::string& path, const std::string& osName);
+
+const char* notifyConfigSaved();
+const char* notifyConfigSaveError();
+const char* notifyPathUpdated();
+
+const char* settingsIntro();
+
+const char* pathListDescriptionLakka();
+
+const char* fileBrowserTitlePrefix();
+std::string fileBrowserNoIni(const std::string& dir);
+
+const char* aboutSubtitle();
+const char* aboutCopyright();
+const char* aboutLinks();
+
+const char* ocBoolLabel(const std::string& key);
+const char* ocFreqLabel(const std::string& key);
+const char* ocVoltageLabel(const std::string& key);
+
+} // namespace I18n