The project structure was refactory to separated the TeX code (in the folder tex) and the documentation resources (in the folder doc).
Add a subtitle= option to the \part command
This lets you specify a subtitle for a part section.
Example: \part[subtitle=My Subtitle]{Part Title}
Improve multilingual file inclusion
The command \localized\input{f} now looks for a file whose name includes the current language code.
f_<lang>.tex (where <lang> is the active language code, e.g. fr for French). f.tex.\localized\input{introduction} attempts to load introduction_fr.tex; if not found, it loads introduction.tex.Fixing the text for the outline slides' titles
The command \outlinetitlelabel is defined for ensuring that the "Outline" is correctly translated in the frame's titles.
Fixing the definition of the localized labels The names of the languages are fixed to have a correct localized label.