feat(translation): add backward compatibility for side-by-side dual P…

…DF option

babeldoc/translation_config.py

@@ -37,6 +37,7 @@ def __init__(
         enhance_compatibility: bool = False,  # 增强兼容性模式
         report_interval: float = 0.1,  # Progress report interval in seconds
         min_text_length: int = 5,  # Minimum text length to translate
+        use_side_by_side_dual: bool = True,  # 是否使用拼版式双语 PDF（并排显示原文和译文） 向下兼容选项，已停用。
         use_alternating_pages_dual: bool = False,  # 是否使用交替页式双语 PDF（交替显示原文和译文）
     ):
         self.input_file = input_file
@@ -64,6 +65,11 @@ def __init__(
         self.report_interval = report_interval
         self.min_text_length = min_text_length
         self.use_alternating_pages_dual = use_alternating_pages_dual
+
+        # for backward compatibility
+        if use_side_by_side_dual is False and use_alternating_pages_dual is False:
+            self.use_alternating_pages_dual = True
+
         if progress_monitor:
             if progress_monitor.cancel_event is None:
                 progress_monitor.cancel_event = threading.Event()