Add language selection to the ChatGUI first-run setup

tools/chatgui/transport/node_config.h

@@ -55,6 +55,7 @@ public:
     QString debugLevel() const { return m_debugLevel; }
     QString debugCategories() const { return m_debugCategories; }
     QString language() const { return m_language; }
+    void setLanguage(const QString& language) { m_language = language; }
 
     /* Config validation errors (collected during load()) */
     bool hasErrors() const { return !m_errors.isEmpty(); }