Facebook JSON badly encoded

I can indeed confirm that the Facebook download data is incorrectly encoded; a Mojibake. The original data is UTF-8 encoded but was decoded as Latin -1 instead. I’ll make sure to file a bug report.

In the meantime, you can repair the damage in two ways:

1. Decode the data as JSON, then re-encode any strings as Latin-1, decode again as UTF-8:

   >>> import json
   >>> data = r'"Rados\u00c5\u0082aw"'
   >>> json.loads(data).encode('latin1').decode('utf8')
   'Radosław'
   

2. Load the data as binary, replace all \u00hh sequences with the byte the last two hex digits represent, decode as UTF-8 and then decode as JSON:

   import re
   from functools import partial
   
   fix_mojibake_escapes = partial(
        re.compile(rb'\\u00([\da-f]{2})').sub,
        lambda m: bytes.fromhex(m.group(1).decode()))
   
   with open(os.path.join(subdir, file), 'rb') as binary_data:
       repaired = fix_mojibake_escapes(binary_data.read())
   data = json.loads(repaired.decode('utf8'))
   

   From your sample data this produces:

   {'content': 'No to trzeba ostatnie treningi zrobić xD',
    'sender_name': 'Radosław',
    'timestamp': 1524558089,
    'type': 'Generic'}