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

@@ -8,7 +8,7 @@ multilinguality:
 task_ids:
 - language-modeling
 - masked-language-modeling
-languages:
+language:
 - 'no'
 - af
 - am
@@ -119,7 +119,7 @@ size_categories:
 - 1K<n<10K
 - 1M<n<10M
 - n<1K
-licenses:
+license:
 - unknown
 paperswithcode_id: opus-100
 configs: