I may as well use this thread to document my progress 
:
In the above linked topic I couldn’t figure out the meaning of those lines :
for a page to answer truthfully to IsMenuCurrent etc. it must be a member of that menu (as: It needs a menu definition in frontmatter).
This topic gave me the answer and I tried to apply it but to no avail… I guess it may have something to do with the fact that my site uses multilingual menus and so my menus definitions are different in my config file.
pachacroute