Rename languages to langage + remove duplicate code in tests

[Wauplin]

Fix CI (currently broken). Also fix #1130.
And also updated tests/documentation + tried to remove some duplicate code in tests.

For context, it seems that most datasets have now a language tag instead of languages:

List datasets with language:en
1750
List datasets with language:fr
196
List datasets with languages:en
7
List datasets with languages:fr
1

(cc @zaidalyafeai in the end I worked on this issue as our CI was broken because of it)

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[codecov]

Codecov Report

Base: 57.45% // Head: 84.77% // Increases project coverage by +27.31% 🎉

Coverage data is based on head (f04a7f6) compared to base (6e4cc64).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1169       +/-   ##
===========================================
+ Coverage   57.45%   84.77%   +27.31%     
===========================================
  Files          42       42               
  Lines        4196     4196               
===========================================
+ Hits         2411     3557     +1146     
+ Misses       1785      639     -1146     

Impacted Files	Coverage Δ
src/huggingface_hub/hf_api.py	87.79% <100.00%> (+44.67%)	⬆️
src/huggingface_hub/utils/endpoint_helpers.py	97.84% <100.00%> (+53.76%)	⬆️
src/huggingface_hub/repository.py	79.44% <0.00%> (+0.86%)	⬆️
src/huggingface_hub/_commit_api.py	93.18% <0.00%> (+4.54%)	⬆️
src/huggingface_hub/utils/_chunk_utils.py	100.00% <0.00%> (+7.14%)	⬆️
src/huggingface_hub/utils/_runtime.py	89.85% <0.00%> (+7.24%)	⬆️
src/huggingface_hub/utils/_deprecation.py	95.55% <0.00%> (+10.00%)	⬆️
src/huggingface_hub/utils/_hf_folder.py	100.00% <0.00%> (+11.53%)	⬆️
src/huggingface_hub/utils/_validators.py	100.00% <0.00%> (+13.63%)	⬆️
src/huggingface_hub/community.py	91.35% <0.00%> (+17.28%)	⬆️
... and 20 more

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[julien-c]

For context, it seems that most datasets have now a language tag instead of languages:

Yes we aligned datasets metadata to models' a few months ago. cc @huggingface/datasets

[LysandreJik]

Yes, LGTM!

[osanseviero]

Thanks a lot!