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

@@ -17,6 +17,9 @@ dataset_info:
     num_examples: 2000
   download_size: 103080661
   dataset_size: 175414492
+language:
+- en
+- de
 ---
 # Dataset Card for "opus-law-en-de-new"