Add multi-language support in book.yml used to generate AGL documentation.

Multi-language will be based on suffixed keys for example:

 "title" : "This is the title of a doc section",
 "title_fr": "Ceci est un titre d'une section de documentation",
 ...

Same logic (suffixed keys) must be also implemented for APIs documentation.

Also introduce a meta tag URL prefix (eg %lang%) that can be used in yaml files.