readme: add language tag

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

@@ -15,4 +15,6 @@ dataset_info:
     num_examples: 517
   download_size: 5684644
   dataset_size: 9511061
----
+language:
+- de
+---