[model_cards] Add language metadata to existing model cards

This will enable filtering on language (amongst other tags) on the website

cc @loretoparisi, @stefan-it, @HenrykBorzymowski, @marma

model_cards/henryk/bert-base-multilingual-cased-finetuned-dutch-squad2/README.md

@@ -1,3 +1,7 @@
+---
+language: dutch
+---
+
 # Multilingual + Dutch SQuAD2.0
 
 This model is the multilingual model provided by the Google research team with a fine-tuned dutch Q&A downstream task.