Fix main.py to correctly build JSON from english/german files

[kunghun-jeong]

a) What line of code you changed

• path_to_file_list (around line 5–10):

  • Changed open(path, 'w') and return lines
    to reading the file in 'r' mode and returning
    a list of lines without newline characters.

• train_file_list_to_json (around line 15–30):

  • Removed manual string concatenation with template_start/mid/end
    and now use json.dumps({"English": en, "German": de}, ensure_ascii=False)
    to generate valid JSON strings.

• write_file_list (around line 30–40):

  • Changed opening the file in 'r' mode and writing only '\n'
    to opening in 'w' mode and writing each element of file_list
    followed by '\n'.

• main (around line 40+):

  • Fixed function calls so that english.txt / german.txt are first
    read by path_to_file_list, and then passed into
    train_file_list_to_json, and the result is written
    to concated.json using write_file_list.

b) Why it is a wrong code

• path_to_file_list opened the file in write mode and never read it,
  returning an undefined variable lines, so it could not work as
  a function that “reads a file and returns a list of lines”.

• train_file_list_to_json did not produce valid JSON format and also
  used only "German" as a key in the template, which does not match
  the required {"English": "...", "German": "..."} structure.

• write_file_list opened the output file in read mode and wrote only
  newline characters, so the result file did not contain any actual data.

• In main, the functions were called with wrong arguments
  (e.g., passing a path string into train_file_list_to_json) so the
  script could not correctly read input files and build concated.json.