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

@@ -21,4 +21,7 @@ configs:
   data_files:
   - split: train
     path: data/train-*
----
+language:
+- si
+- en
+---