readme: add language tags

Hi,

as part of the #Hacktoberfest [2023](https://github.com/huggingface/hub-docs/issues/986) this MR extends the dataset metadata and adds a language tag.

Happy merging!

README.md

@@ -13,6 +13,9 @@ dataset_info:
     num_examples: 9759125
   download_size: 1926773979
   dataset_size: 4189844561
+language:
+- ar
+- en
 ---
 # Dataset Card for "un_multi-ar-en"