[model_cards] Switch all languages codes to ISO-639-{1,2,3}

model_cards/xlm-roberta-large-finetuned-conll03-german-README.md

 ---
-language: german
+language: de
 ---
 
 ## xlm-roberta-large-finetuned-conll03-german

model_cards/yjernite/bart_eli5/README.md

 ---
-language: english
+language: en
 license: apache-2.0
 datasets:
 - eli5

model_cards/youscan/ukr-roberta-base/README.md

 ---
 language:
-- ukrainian
+- uk
 ---
 
 # ukr-roberta-base