Some languages in wikipedia dataset are not loading 
 Hi,

I am working with the `wikipedia` dataset and I have a script that goes over 92 of the available languages in that dataset. So far I have detected that `ar`, `af`, `an` are not loading. Other languages like `fr` and `en` are working fine. Here's how I am loading them:

```
import nlp

langs = ['ar'. 'af', 'an']

for lang in langs:
    data = nlp.load_dataset('wikipedia', f'20200501.{lang}', beam_runner='DirectRunner', split='train') 
    print(lang, len(data))
```

Here's what I see for 'ar' (it gets stuck there):
```
Downloading and preparing dataset wikipedia/20200501.ar (download: Unknown size, generated: Unknown size, post-processed: Unknown sizetotal: Unknown size) to /home/gaguilar/.cache/huggingface/datasets/wikipedia/20200501.ar/1.0.0/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50...
```

Note that those languages are indeed in the list of expected languages. Any suggestions on how to work around this? Thanks! 
 > About the date issue, I think it's possible to use another date with
> ```python
> load_dataset("wikipedia", language="es", date="...", beam_runner="...")
> ```

I tried your suggestion about the date and the function does not accept the language and date keywords. I tried both on `nlp` v0.4 and the new `datasets` library (v1.0.2):
```
load_dataset("wikipedia", language="es", date="20200601", beam_runner='DirectRunner', split='train')
```
For now, my quick workaround to keep things moving was to simply change the date inside the library at this line: [https://github.com/huggingface/datasets/blob/master/datasets/wikipedia/wikipedia.py#L403](https://github.com/huggingface/datasets/blob/master/datasets/wikipedia/wikipedia.py#L403)

Note that the date and languages are valid: [https://dumps.wikimedia.org/eswiki/20200601/dumpstatus.json](https://dumps.wikimedia.org/eswiki/20200601/dumpstatus.json)

Any suggestion is welcome :) @lhoestq 


## **[UPDATE]**

The workaround I mentioned fetched the data, but then I faced another issue (even the log says to report this as bug):
```
ERROR:root:mwparserfromhell ParseError: This is a bug and should be reported. Info: C tokenizer exited with non-empty token stack.
```

Here's the full stack (which says that there is a key error caused by this key: `KeyError: '000nbsp'`):

```Downloading and preparing dataset wikipedia/20200601.es (download: Unknown size, generated: Unknown size, post-processed: Unknown sizetotal: Unknown size) to /home/gustavoag/.cache/huggingface/datasets/wikipedia/20200601.es/1.0.0/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50...
Downloading: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 74.7k/74.7k [00:00<00:00, 1.53MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 232M/232M [00:48<00:00, 4.75MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 442M/442M [01:39<00:00, 4.44MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 173M/173M [00:33<00:00, 5.12MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 344M/344M [01:14<00:00, 4.59MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 541M/541M [01:59<00:00, 4.52MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 476M/476M [01:31<00:00, 5.18MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 545M/545M [02:02<00:00, 4.46MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 299M/299M [01:01<00:00, 4.89MB/s]
Downloading: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 9.60M/9.60M [00:01<00:00, 4.84MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 423M/423M [01:36<00:00, 4.38MB/s]
WARNING:apache_beam.options.pipeline_options:Discarding unparseable args: ['--lang', 'es', '--date', '20200601', '--tokenizer', 'bert-base-multilingual-cased', '--cache', 'train', 'valid', '--max_dataset_length', '200000', '10000']

ERROR:root:mwparserfromhell ParseError: This is a bug and should be reported. Info: C tokenizer exited with non-empty token stack.
ERROR:root:mwparserfromhell ParseError: This is a bug and should be reported. Info: C tokenizer exited with non-empty token stack.
ERROR:root:mwparserfromhell ParseError: This is a bug and should be reported. Info: C tokenizer exited with non-empty token stack.
ERROR:root:mwparserfromhell ParseError: This is a bug and should be reported. Info: C tokenizer exited with non-empty token stack.
Traceback (most recent call last):
  File "apache_beam/runners/common.py", line 961, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 553, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1095, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/datasets/wikipedia/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50/wikipedia.py", line 500, in _clean_content
    text = _parse_and_clean_wikicode(raw_content, parser=mwparserfromhell)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/datasets/wikipedia/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50/wikipedia.py", line 556, in _parse_and_clean_wikicode
    section_text.append(section.strip_code().strip())
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/mwparserfromhell/wikicode.py", line 643, in strip_code
    stripped = node.__strip__(**kwargs)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/mwparserfromhell/nodes/html_entity.py", line 63, in __strip__
    return self.normalize()
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/mwparserfromhell/nodes/html_entity.py", line 178, in normalize
    return chrfunc(htmlentities.name2codepoint[self.value])
KeyError: '000nbsp'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/raid/data/gustavoag/projects/char2subword/research/preprocessing/split_wiki.py", line 96, in <module>
    main()
  File "/raid/data/gustavoag/projects/char2subword/research/preprocessing/split_wiki.py", line 65, in main
    data = nlp.load_dataset('wikipedia', f'{args.date}.{args.lang}', beam_runner='DirectRunner', split='train')
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/load.py", line 548, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/builder.py", line 462, in download_and_prepare
    self._download_and_prepare(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/builder.py", line 969, in _download_and_prepare
    pipeline_results = pipeline.run()
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/pipeline.py", line 534, in run
    return self.runner.run_pipeline(self, self._options)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/direct/direct_runner.py", line 119, in run_pipeline
    return runner.run_pipeline(pipeline, options)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 172, in run_pipeline
    self._latest_run_result = self.run_via_runner_api(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 183, in run_via_runner_api
    return self.run_stages(stage_context, stages)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 338, in run_stages
    stage_results = self._run_stage(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 512, in _run_stage
    last_result, deferred_inputs, fired_timers = self._run_bundle(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 556, in _run_bundle
    result, splits = bundle_manager.process_bundle(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 940, in process_bundle
    for result, split_result in executor.map(execute, zip(part_inputs,  # pylint: disable=zip-builtin-not-iterating
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/concurrent/futures/_base.py", line 611, in result_iterator
    yield fs.pop().result()
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/concurrent/futures/_base.py", line 439, in result
    return self.__get_result()
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
    raise self._exception
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/utils/thread_pool_executor.py", line 44, in run
    self._future.set_result(self._fn(*self._fn_args, **self._fn_kwargs))
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 932, in execute
    return bundle_manager.process_bundle(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 837, in process_bundle
    result_future = self._worker_handler.control_conn.push(process_bundle_req)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/portability/fn_api_runner/worker_handlers.py", line 352, in push
    response = self.worker.do_instruction(request)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/worker/sdk_worker.py", line 479, in do_instruction
    return getattr(self, request_type)(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/worker/sdk_worker.py", line 515, in process_bundle
    bundle_processor.process_bundle(instruction_id))
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/worker/bundle_processor.py", line 977, in process_bundle
    input_op_by_transform_id[element.transform_id].process_encoded(
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/apache_beam/runners/worker/bundle_processor.py", line 218, in process_encoded
    self.output(decoded_value)
  File "apache_beam/runners/worker/operations.py", line 330, in apache_beam.runners.worker.operations.Operation.output
  File "apache_beam/runners/worker/operations.py", line 332, in apache_beam.runners.worker.operations.Operation.output
  File "apache_beam/runners/worker/operations.py", line 195, in apache_beam.runners.worker.operations.SingletonConsumerSet.receive
  File "apache_beam/runners/worker/operations.py", line 670, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/worker/operations.py", line 671, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/common.py", line 963, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 1030, in apache_beam.runners.common.DoFnRunner._reraise_augmented
  File "apache_beam/runners/common.py", line 961, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 553, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1122, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "apache_beam/runners/worker/operations.py", line 195, in apache_beam.runners.worker.operations.SingletonConsumerSet.receive
  File "apache_beam/runners/worker/operations.py", line 670, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/worker/operations.py", line 671, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/common.py", line 963, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 1030, in apache_beam.runners.common.DoFnRunner._reraise_augmented
  File "apache_beam/runners/common.py", line 961, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 553, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1122, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "apache_beam/runners/worker/operations.py", line 195, in apache_beam.runners.worker.operations.SingletonConsumerSet.receive
  File "apache_beam/runners/worker/operations.py", line 670, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/worker/operations.py", line 671, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/common.py", line 963, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 1045, in apache_beam.runners.common.DoFnRunner._reraise_augmented
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/future/utils/__init__.py", line 446, in raise_with_traceback
    raise exc.with_traceback(traceback)
  File "apache_beam/runners/common.py", line 961, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 553, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1095, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/datasets/wikipedia/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50/wikipedia.py", line 500, in _clean_content
    text = _parse_and_clean_wikicode(raw_content, parser=mwparserfromhell)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/nlp/datasets/wikipedia/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50/wikipedia.py", line 556, in _parse_and_clean_wikicode
    section_text.append(section.strip_code().strip())
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/mwparserfromhell/wikicode.py", line 643, in strip_code
    stripped = node.__strip__(**kwargs)
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/mwparserfromhell/nodes/html_entity.py", line 63, in __strip__
    return self.normalize()
  File "/home/gustavoag/anaconda3/envs/pytorch/lib/python3.8/site-packages/mwparserfromhell/nodes/html_entity.py", line 178, in normalize
    return chrfunc(htmlentities.name2codepoint[self.value])
KeyError: "000nbsp [while running 'train/Clean content']"```