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 <[email protected]>

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

README.md

@@ -1,6 +1,6 @@
 ---
 pretty_name: Extreme Summarization (XSum)
-languages:
+language:
 - en
 paperswithcode_id: xsum
 task_categories: