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

@@ -19,4 +19,6 @@ dataset_info:
     num_examples: 518
   download_size: 8513229
   dataset_size: 17600335
----
+language:
+- nl
+---