Fix streaming xnli

This code was failing

```python

In [1]: from datasets import load_dataset

In [2]: 
   ...:         ds = load_dataset("xnli", "all_languages", split="test", streaming=True)
   ...: 
   ...:         sample_data = next(iter(ds))["premise"]  # pick up one data
   ...:         input_text = list(sample_data.values())
```

```
File ~/hf/datasets/src/datasets/features/translation.py:104, in TranslationVariableLanguages.encode_example(self, translation_dict)
    102     return translation_dict
    103 elif self.languages and set(translation_dict) - lang_set:
--> 104     raise ValueError(
    105         f'Some languages in example ({", ".join(sorted(set(translation_dict) - lang_set))}) are not in valid set ({", ".join(lang_set)}).'
    106     )
    108 # Convert dictionary into tuples, splitting out cases where there are
    109 # multiple translations for a single language.
    110 translation_tuples = []

ValueError: Some languages in example (language, translation) are not in valid set (ur, fr, hi, sw, vi, el, de, th, en, tr, zh, ar, bg, ru, es).
```

because in streaming mode we expect features encode methods to be no-ops if the example is already encoded.

I fixed `TranslationVariableLanguages` to account for that

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6503). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005003 / 0.011353 (-0.006350) | 0.003020 / 0.011008 (-0.007988) | 0.061370 / 0.038508 (0.022862) | 0.050996 / 0.023109 (0.027887) | 0.243434 / 0.275898 (-0.032464) | 0.266317 / 0.323480 (-0.057163) | 0.003888 / 0.007986 (-0.004098) | 0.002607 / 0.004328 (-0.001721) | 0.047541 / 0.004250 (0.043290) | 0.037933 / 0.037052 (0.000881) | 0.259695 / 0.258489 (0.001206) | 0.279374 / 0.293841 (-0.014467) | 0.027258 / 0.128546 (-0.101288) | 0.010184 / 0.075646 (-0.065462) | 0.207412 / 0.419271 (-0.211860) | 0.034978 / 0.043533 (-0.008554) | 0.247871 / 0.255139 (-0.007267) | 0.265273 / 0.283200 (-0.017927) | 0.017886 / 0.141683 (-0.123796) | 1.090451 / 1.452155 (-0.361704) | 1.152034 / 1.492716 (-0.340682) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.094383 / 0.018006 (0.076377) | 0.301151 / 0.000490 (0.300661) | 0.000211 / 0.000200 (0.000011) | 0.000049 / 0.000054 (-0.000005) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.018927 / 0.037411 (-0.018484) | 0.062152 / 0.014526 (0.047626) | 0.072177 / 0.176557 (-0.104380) | 0.119792 / 0.737135 (-0.617343) | 0.073333 / 0.296338 (-0.223005) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.282671 / 0.215209 (0.067462) | 2.721148 / 2.077655 (0.643494) | 1.472689 / 1.504120 (-0.031431) | 1.355226 / 1.541195 (-0.185969) | 1.375935 / 1.468490 (-0.092556) | 0.562600 / 4.584777 (-4.022177) | 2.364046 / 3.745712 (-1.381666) | 2.714984 / 5.269862 (-2.554878) | 1.738413 / 4.565676 (-2.827263) | 0.062564 / 0.424275 (-0.361711) | 0.004964 / 0.007607 (-0.002643) | 0.341300 / 0.226044 (0.115255) | 3.345187 / 2.268929 (1.076259) | 1.857822 / 55.444624 (-53.586803) | 1.581002 / 6.876477 (-5.295475) | 1.585919 / 2.142072 (-0.556153) | 0.640105 / 4.805227 (-4.165122) | 0.117880 / 6.500664 (-6.382784) | 0.042032 / 0.075469 (-0.033437) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.962701 / 1.841788 (-0.879086) | 11.309251 / 8.074308 (3.234943) | 10.462520 / 10.191392 (0.271128) | 0.127399 / 0.680424 (-0.553025) | 0.014549 / 0.534201 (-0.519652) | 0.297017 / 0.579283 (-0.282266) | 0.266152 / 0.434364 (-0.168212) | 0.349252 / 0.540337 (-0.191085) | 0.457015 / 1.386936 (-0.929921) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005341 / 0.011353 (-0.006012) | 0.003108 / 0.011008 (-0.007900) | 0.048862 / 0.038508 (0.010353) | 0.053354 / 0.023109 (0.030245) | 0.274499 / 0.275898 (-0.001399) | 0.296698 / 0.323480 (-0.026782) | 0.003974 / 0.007986 (-0.004012) | 0.002631 / 0.004328 (-0.001697) | 0.048013 / 0.004250 (0.043762) | 0.040416 / 0.037052 (0.003363) | 0.276581 / 0.258489 (0.018092) | 0.301296 / 0.293841 (0.007455) | 0.029049 / 0.128546 (-0.099497) | 0.010253 / 0.075646 (-0.065393) | 0.057157 / 0.419271 (-0.362114) | 0.031830 / 0.043533 (-0.011703) | 0.274341 / 0.255139 (0.019202) | 0.292583 / 0.283200 (0.009383) | 0.018449 / 0.141683 (-0.123234) | 1.145099 / 1.452155 (-0.307055) | 1.192958 / 1.492716 (-0.299758) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.091596 / 0.018006 (0.073590) | 0.300917 / 0.000490 (0.300427) | 0.000225 / 0.000200 (0.000025) | 0.000054 / 0.000054 (-0.000001) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.021657 / 0.037411 (-0.015754) | 0.068464 / 0.014526 (0.053938) | 0.079869 / 0.176557 (-0.096687) | 0.117523 / 0.737135 (-0.619613) | 0.081257 / 0.296338 (-0.215082) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.294876 / 0.215209 (0.079667) | 2.879372 / 2.077655 (0.801718) | 1.619887 / 1.504120 (0.115767) | 1.482154 / 1.541195 (-0.059041) | 1.494656 / 1.468490 (0.026166) | 0.558914 / 4.584777 (-4.025862) | 2.420948 / 3.745712 (-1.324765) | 2.728992 / 5.269862 (-2.540869) | 1.722135 / 4.565676 (-2.843542) | 0.062182 / 0.424275 (-0.362093) | 0.004933 / 0.007607 (-0.002674) | 0.342759 / 0.226044 (0.116715) | 3.424083 / 2.268929 (1.155154) | 1.950673 / 55.444624 (-53.493951) | 1.683126 / 6.876477 (-5.193351) | 1.673135 / 2.142072 (-0.468937) | 0.633711 / 4.805227 (-4.171516) | 0.114898 / 6.500664 (-6.385766) | 0.040332 / 0.075469 (-0.035137) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.975102 / 1.841788 (-0.866685) | 11.975731 / 8.074308 (3.901423) | 10.961103 / 10.191392 (0.769711) | 0.131152 / 0.680424 (-0.549272) | 0.016268 / 0.534201 (-0.517933) | 0.285031 / 0.579283 (-0.294252) | 0.279556 / 0.434364 (-0.154808) | 0.324183 / 0.540337 (-0.216154) | 0.571404 / 1.386936 (-0.815532) |

</details>
</details>

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