process .opus files (for Multilingual Spoken Words)
@lhoestq  I still have problems with processing `.opus` files with `soundfile` so I actually cannot fully check that it works but it should... Maybe this should be investigated in case of someone else would also have problems with that.

Also, as the data is in a private repo on the hub (before we come to a decision about audio data privacy), the needed checks cannot be done right now. @lhoestq I check the data redownloading for configs sharing the same languages, you were right: the data is downloaded once for each language. But samples are generated from scratch each time. Is it a supposed behavior?   > But samples are generated from scratch each time. Is it a supposed behavior?

Yea that's the way it works right now, because we generate one arrow file per configuration. Since changing the languages creates a new configuration, then it generates a new arrow file.
Opus files requires `libsndfile>=1.0.30`. Add check for this version and tests.


**outdated:**
Add [Multillingual Spoken Words dataset](https://mlcommons.org/en/multilingual-spoken-words/)

You can specify multiple languages for downloading 😌:
```python
ds = load_dataset("datasets/ml_spoken_words", languages=["ar", "tt"])
```

1. I didn't take into account that each time you pass a set of languages the data for a specific language is downloaded even if it was downloaded before (since these are custom configs like `ar+tt` and `ar+tt+br`. Maybe that wasn't a good idea?
2. The script will have to be slightly changed after merge of https://github.com/huggingface/datasets/pull/3664 
2. Just can't figure out what wrong with dummy files... 😞 Maybe we should get rid of them at some point 😁