Update README.md to include ISO codes for available languages

Source:

Wiki: https://en.wikipedia.org/wiki/Bengali_language
Hugging Face: https://huggingface.co/languages

README.md

@@ -3,4 +3,7 @@ task_categories:
 - translation
 size_categories:
 - 1M<n<10M
+language:
+- en
+- bn
 ---