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

@@ -53,6 +53,8 @@ dataset_info:
     num_examples: 2
   download_size: 0
   dataset_size: 38104
+language:
+- de
 ---
 # Dataset Card for "alpaca-german-validation"