Where can we find the mapping of special tokens?

[Rooders]

Thanks for your great work, and I have some questions about the mapping of special tokens, like the separated token <extra_id_0> and the language symbol <extra_id_5>, I want to know the <extra_id_5> means English? And if my training data is German-to-French, what are the language symbols of German and French? where I can find the mapping of these special tokens (symbols)?