Align more metadata with other repo types (models,spaces) (#4607)

* language(s) and license(s) for consistency

* `licenses:` => `license:`

* `languages:` => `language:`

* backward-compat

* make style

* set default values for deprecated languages and licenses

Co-authored-by: Quentin Lhoest <lhoest.q@gmail.com>

Commit from https://github.com/huggingface/datasets/commit/a5192964dc4b76ee5c03593c11ee56f29bbd688d

README.md

@@ -5,7 +5,7 @@ multilinguality:
 - multilingual
 task_ids:
 - text-classification-other-language-identification
-languages:
+language:
 - hi
 - awa
 - bho
@@ -17,7 +17,7 @@ source_datasets:
 - original
 size_categories:
 - 10K<n<100K
-licenses:
+license:
 - unknown
 paperswithcode_id: null
 pretty_name: ilist