It's a bit difficult to diagnose without seeing the code, but I'll take a stab at it and suggest that it's most likely to be a language issue.

Let's assume this AddIn adds menu items. The popular way to do this is to refer to the commandbar you want to add the menu to by it's name, so for the main menu bar, you would start with:[VBA]With Application.CommandBars("Worksheet Menu Bar")
'add a menu then some controls, etc
End With[/VBA]
The thing with the Germans and Portuguese is, they got a different word for everything... and while it's possible to order a beer by a process of gesturing and pointing for a couple of minutes, referring to Excel objects requires a little more precision. So unless the German/Portuguese for "Worksheet Menu Bar" is, in fact, "Worksheet Menu Bar", there'll be a problem.
This would explain the Error 5.

The solution is to refer to the native commandbars and controls using their ID property
e.g for the Worksheet Menu Bar, it's 265

The invalid argument is more difficult to get to without debugging, but following the same logic, it may be that an empty string (where some menu text should be) is being passed ? Would need to see code to trace that one.

Hope that helps