[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/dbmdz/bert-base-italian-uncased/README.md

@@ -1,3 +1,7 @@
+---
+language: italian
+---
+
 # 🤗 + 📚 dbmdz BERT models
 
 In this repository the MDZ Digital Library team (dbmdz) at the Bavarian State