Hi Dani thanks for replying,
The problem was solved, but it took some time to realize what was causing this problem.
Following what you said about changing the string in the "lang" property in the mobile.php file to:
The title was being displayed as "agenda", so after doing some trial and error it turns out that I had to replace 'parentagenda' with 'local_parentagenda' since the plugin was a "local plugin" and the one provided in the documentation is under "mod" (mod_certificate).
So that's about it, now it's being displayed as "Agenda" in the menu and it's working perfectly.
Thanks for the help Dani