App.MenuBar and the window’s MenuBar are separate instances, even when they come from the same menu design. On macOS, the active window supplies the visible menu captions, including About, so changing the App instance doesn’t change that window’s menu.
Try putting this in the window’s Opening event:
Self.MenuBar.FindItem("aboutItem").Text = "Test"
For localisation, replace "Test" with your resource lookup:
Self.MenuBar.FindItem("aboutItem").Text = Localise("menu.about")
You’ll also want to translate App.MenuBar for the macOS menu shown when no windows are open.
While investigating this, I found a separate macOS refresh bug: changing captions after the native menu has been installed doesn’t reliably update the displayed menu, particularly items such as About. I’ve logged that here.
Setting the window’s captions in Opening happens before its native menu is installed, so that is the approach to use for initial localisation.