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

@@ -6,6 +6,8 @@ task_categories:
 pretty_name: Movie-Reviews
 size_categories:
 - 10K<n<100K
+language:
+- en
 ---
 # IMDB Movie Reviews