Update Special Language Tokens for PLBART (#19980)
* Update Special Language Tokens for PLBART
* fix format
* making mapping for language codes and updating tests:
* fix format
* fix consistency
* add assert to both tokenizer tests.
* fix format
* Update src/transformers/models/plbart/tokenization_plbart.py
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
* improvin readability, setting self.tgt_lang
* fixing
* readability
Co-authored-by: jordiclive <jordiclive19@imperial.ac.uk>
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
