Fix main.py to generate valid JSON from input files

[hongjunki-405]

Changes:

1. path_to_file_list (around line XX)
   Bug: opened the file with mode 'w' and returned undefined variable lines.
   Fix: open the file in read mode ('r'), read all lines, strip trailing '\n' and return the list.

2. train_file_list_to_json (around line YY)
   Bug: wrong escaping logic and incorrect JSON template

   • file.replace('\\', '\\') did nothing.
   • condition if '/' or '"' in file: was always True.
   • English variable was overwritten by german file.
   • JSON string used wrong keys and wrong order.
     Fix: properly escape backslash, double quote and slash, then build a JSON line as
     -{"English":"...","German":"..."}.

3. write_file_list (around line ZZ)
   Bug: opened the output file with mode 'r' and only wrote '\n' without content.
   Fix: open file in write mode ('w') and write each JSON string followed by a newline.