add language support

The tool support only italian language, add other language require to configure properly each backend provided to the tool.

To add new language, the developer Need to modify the global dictionary as following:

• LANGUAGE_DICT = {"it-IT":"it"}
• LANGUAGE_DICT_PYTTS = {"it-IT":"italian"} in m4b.py
• TITLE_KEYWORD = {"it-IT":"TITOLO", "en":"TITLE"}
• CHAPTER_KEYWORD= {"it-IT":"CAPITOLO", "en":"CHAPTER"} in docx2audio.py

Desiderable: looking for hard coded strings and generalize it with a global dictionary, as previous one.