Implement 'Set Language' UI to persist language changes to page header

src/teletext/ui.py

@@ -298,6 +298,26 @@ class MainWindow(QMainWindow):
         btn_release.clicked.connect(lambda: self.insert_char(0x1F))
         right_layout.addWidget(btn_release)
         
+        right_layout.addSpacing(10)
+
+        # Page Language Setting
+        lang_group_label = QLabel("Page Language:")
+        right_layout.addWidget(lang_group_label)
+        
+        lang_layout = QHBoxLayout()
+        self.lang_combo = QComboBox()
+        self.lang_combo.addItems(self.language_names)
+        self.lang_combo.setToolTip("Select the National Option Character Set for this page.")
+        
+        btn_set_lang = QPushButton("Set")
+        btn_set_lang.setFixedWidth(40)
+        btn_set_lang.clicked.connect(self.apply_language_change)
+        btn_set_lang.setToolTip("Apply this language setting to the page header.")
+        
+        lang_layout.addWidget(self.lang_combo)
+        lang_layout.addWidget(btn_set_lang)
+        right_layout.addLayout(lang_layout)
+        
         right_layout.addStretch()
         
         self.layout.addLayout(center_layout, 1)
@@ -330,6 +350,7 @@ class MainWindow(QMainWindow):
         idx = self.canvas.subset_idx
         if 0 <= idx < len(self.language_names):
             self.language_label.setText(f"Lang: {self.language_names[idx]}")
+            self.lang_combo.setCurrentIndex(idx)
         else:
             self.language_label.setText(f"Lang: Unknown ({idx})")
 
@@ -453,6 +474,73 @@ class MainWindow(QMainWindow):
             self.canvas.update()
             self.update_language_label()
 
+    def apply_language_change(self):
+        if not self.current_page:
+            return
+            
+        idx = self.lang_combo.currentIndex()
+        if idx < 0: return
+        
+        # Update the model
+        self.current_page.language = idx
+        
+        # Also update the session override/viewer to match
+        self.canvas.subset_idx = idx
+        key = (self.current_page.magazine, self.current_page.page_number)
+        self.language_overrides[key] = idx
+        
+        # Patch Row 0 packet data to persist language selection to file
+        # Language bits are in Byte 8 (Control Bits 2): C12, C13, C14
+        # We need to preserve C11 (bit 3 of encoded 4-bit val) which is "Inhibit Display" usually 0
+        
+        # Find Row 0 packet
+        header_packet = None
+        for p in self.current_page.packets:
+            if p.row == 0:
+                header_packet = p
+                break
+        
+        if header_packet and len(header_packet.data) > 8:
+            try:
+                old_val = decode_hamming_8_4(header_packet.data[8])
+                # Encoded nibble structure: D1(b0), D2(b1), D3(b2), D4(b3)
+                # D1 maps to C12
+                # D2 maps to C13
+                # D3 maps to C14
+                # D4 maps to C11
+                
+                # io.py logic for reading:
+                # language = ((c_bits_2 & 1) << 1) | ((c_bits_2 & 2) >> 1) | (c_bits_2 & 4)
+                # i.e. Lang Bit 0 comes from D2, Lang Bit 1 comes from D1, Lang Bit 2 comes from D3
+                
+                # So for writing:
+                # D1 = Lang Bit 1
+                # D2 = Lang Bit 0
+                # D3 = Lang Bit 2
+                
+                l0 = (idx >> 0) & 1
+                l1 = (idx >> 1) & 1
+                l2 = (idx >> 2) & 1
+                
+                d1 = l1
+                d2 = l0
+                d3 = l2
+                d4 = (old_val >> 3) & 1 # Preserve C11
+                
+                new_val = d1 | (d2 << 1) | (d3 << 2) | (d4 << 3)
+                
+                header_packet.data[8] = encode_hamming_8_4(new_val)
+                self.set_modified(True)
+                self.status_label.setText(f"Language set to {self.language_names[idx]} (saved to header).")
+            except Exception as e:
+                self.status_label.setText(f"Error setting language: {e}")
+        else:
+            self.status_label.setText("No header packet found to update.")
+            
+        self.canvas.redraw()
+        self.canvas.update()
+        self.update_language_label()
+
     def prev_subpage(self):
         count = self.subpage_combo.count()
         if count <= 1: return