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

@@ -11,6 +11,9 @@ dataset_info:
     num_examples: 664
   download_size: 305819
   dataset_size: 494409
+language:
+- cs
+- en
 ---
 # Dataset Card for "wmt20-cs-en"