[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/Musixmatch/umberto-commoncrawl-cased-v1/README.md

@@ -1,3 +1,7 @@
+---
+language: italian
+---
+
 # UmBERTo Commoncrawl Cased
 
 [UmBERTo](https://github.com/musixmatchresearch/umberto) is a Roberta-based Language Model trained on large Italian Corpora and uses two innovative approaches: SentencePiece and Whole Word Masking. Now available at [github.com/huggingface/transformers](https://huggingface.co/Musixmatch/umberto-commoncrawl-cased-v1)

model_cards/Musixmatch/umberto-wikipedia-uncased-v1/README.md

@@ -1,3 +1,7 @@
+---
+language: italian
+---
+
 # UmBERTo Wikipedia Uncased
 
 [UmBERTo](https://github.com/musixmatchresearch/umberto) is a Roberta-based Language Model trained on large Italian Corpora and uses two innovative approaches: SentencePiece and Whole Word Masking. Now available at [github.com/huggingface/transformers](https://huggingface.co/Musixmatch/umberto-commoncrawl-cased-v1)