Hi Ofi Khan,
I finally managed to fix the problem myself; this translation problem that appeared in the menu subpages was due to the fact that the original links in the main menu (in English) were set to all languages, not just English...
I'm not sure why it worked for the main pages and not for the sub-pages, but once the language was set to English, everything went back to normal in the other language menus and my translated alias were taken into account for sub-pages too.
Anyway, problem solved, thanks.