fix multiple language downloading in mC4
{'login': 'polinaeterna', 'id': 16348744, 'node_id': 'MDQ6VXNlcjE2MzQ4NzQ0', 'avatar_url': 'https://avatars.githubusercontent.com/u/16348744?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/polinaeterna', 'html_url': 'https://github.com/polinaeterna', 'followers_url': 'https://api.github.com/users/polinaeterna/followers', 'following_url': 'https://api.github.com/users/polinaeterna/following{/other_user}', 'gists_url': 'https://api.github.com/users/polinaeterna/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/polinaeterna/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/polinaeterna/subscriptions', 'organizations_url': 'https://api.github.com/users/polinaeterna/orgs', 'repos_url': 'https://api.github.com/users/polinaeterna/repos', 'events_url': 'https://api.github.com/users/polinaeterna/events{/privacy}', 'received_events_url': 'https://api.github.com/users/polinaeterna/received_events', 'type': 'User', 'site_admin': False}
[]
[]
The CI failure is unrelated to your PR and fixed on master, merging :)
If we try to access multiple languages of the [mC4 dataset](https://github.com/huggingface/datasets/tree/master/datasets/mc4), it will throw an error. For example, if we do 
```python
mc4_subset_two_langs = load_dataset("mc4", languages=["st", "su"])
```
we got
```
FileNotFoundError: Couldn't find file at https://huggingface.co/datasets/allenai/c4/resolve/1ddc917116b730e1859edef32896ec5c16be51d0/multilingual/c4-st+su.tfrecord-00000-of-00002.json.gz
```
Now it should work. Check it (from the root dir of a project): 
```python
mc4_subset_two_langs = load_dataset("./datasets/mc4/", languages=["st", "su"])
```
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/3594/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/3594', 'html_url': 'https://github.com/huggingface/datasets/pull/3594', 'diff_url': 'https://github.com/huggingface/datasets/pull/3594.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/3594.patch', 'merged_at': '2022-01-18T19:10:22Z'}