ModuleNotFoundError: No module named 'apache_beam', when specific languages.
Hi !

Apache Beam is a framework used to define data transformation pipelines. These pipeline can then be run in many runtimes: DataFlow, Spark, Flink, etc. There also exist a local runner called the DirectRunner.
Wikipedia is a dataset that requires some parsing, so to allow the processing to be run on this kind of runtime we're using Apache Beam.

At Hugging Face we've already processed certain versions of wikipedia (the `20200501.en` one for example) so that users can directly download the processed version instead of using Apache Beam to process it.
However for the japanese language we haven't processed it so you'll have to run the processing on your side.
So you do need Apache Beam to process `20200501.ja`.

You can install Apache Beam with
```
pip install apache-beam
```

I think we can probably improve the error message to let users know of this subtlety.
What #498 implied is that Apache Beam is not needed when you process a dataset that doesn't use Apache Beam.
```py
import datasets
wiki = datasets.load_dataset('wikipedia', '20200501.ja', cache_dir='./datasets')
```
then `ModuleNotFoundError: No module named 'apache_beam'` happend.

The error doesn't appear when it's '20200501.en'.
I don't know Apache Beam, but according to #498 it isn't necessary when it's saved to local. is it correct?
ModuleNotFoundError: No module named 'apache_beam', when specific languages. 
 ```py
import datasets
wiki = datasets.load_dataset('wikipedia', '20200501.ja', cache_dir='./datasets')
```
then `ModuleNotFoundError: No module named 'apache_beam'` happend.

The error doesn't appear when it's '20200501.en'.
I don't know Apache Beam, but according to #498 it isn't necessary when it's saved to local. is it correct? 
 Hi !

Apache Beam is a framework used to define data transformation pipelines. These pipeline can then be run in many runtimes: DataFlow, Spark, Flink, etc. There also exist a local runner called the DirectRunner.
Wikipedia is a dataset that requires some parsing, so to allow the processing to be run on this kind of runtime we're using Apache Beam.

At Hugging Face we've already processed certain versions of wikipedia (the `20200501.en` one for example) so that users can directly download the processed version instead of using Apache Beam to process it.
However for the japanese language we haven't processed it so you'll have to run the processing on your side.
So you do need Apache Beam to process `20200501.ja`.

You can install Apache Beam with
```
pip install apache-beam
```

I think we can probably improve the error message to let users know of this subtlety.
What #498 implied is that Apache Beam is not needed when you process a dataset that doesn't use Apache Beam.