code: def set_main_and_cell_language(
      metadata, cells, ext, custom_cell_magics
  ):
      """Set main language for the given collection of cells, and
      use magics for cells that use other languages"""
      main_language = default_language_from_metadata_and_ext(
          metadata, ext
      )
  
      if main_language is None:
          languages = {"python": 0.5}
          for cell in cells:
              if "language" in cell["metadata"]:
                  language = usual_language_name(
                      cell["metadata"]["language"]
                  )
                  languages[language] = languages.get(language, 0.0) + 1
  
          main_language = max(languages, key=languages.get)
  
      # save main language when no kernel is set
      if "language" not in metadata.get("kernelspec", {}) and cells:
          metadata.setdefault("jupytext", {})[
              "main_language"
          ] = main_language
  
      # Remove 'language' meta data and add a magic if not main language
      for cell in cells:
          if "language" in cell["metadata"]:
              language = cell["metadata"]["language"]
              if language == main_language:
                  cell["metadata"].pop("language")
                  continue
  
              if usual_language_name(language) == main_language:
                  continue
  
              if (
                  language in _JUPYTER_LANGUAGES
                  or language in custom_cell_magics
              ):
                  cell["metadata"].pop("language")
                  magic = "%%" if main_language != "csharp" else "#!"
                  if "magic_args" in cell["metadata"]:
                      magic_args = cell["metadata"].pop("magic_args")
                      cell["source"] = (
                          f"{magic}{language} {magic_args}\n"
                          + cell["source"]
                      )
                  else:
                      cell["source"] = (
                          f"{magic}{language}\n" + cell["source"]
                      )
  
label_desc: Use image node_id 4 for calling a global function with example usage: set_main_and_cell_language(metadata, cells, ext, custom_cell_magics) without return types