Can confirm with a different language too. However there's another string on the same page that also lacks a domain string, and displays the translated text as expected. How odd. CCing some i18n people who may understand what's going on here.

I think the problem happens because the QML file shows the text without setting the correct translation domain.
The translation actually exists in the system, but QML doesn’t know which file to use, so it keeps showing the English text.

File:
plasma-desktop-master/desktoppackage/contents/configuration/ConfigurationShortcuts.qml

Relevant code snippet:

KCM.SimpleKCM {
    title: i18n("Shortcuts")

    Kirigami.FormLayout {
        KeySequenceItem {
            Kirigami.FormData.label: i18nc("@action:button set keyboard shortcut for", "Activate widget as if clicked:")
        }
    }
}

To fix the issue, it would be necessary to add either:

Kirigami.LocaleHelper {
    id: localeHelper
    domain: "plasma_shell_org.kde.plasma.desktop"
}

or:

KCM.SimpleKCM {
    KCM.ConfigModule {
        translationDomain: "plasma_shell_org.kde.plasma.desktop"
    }
    ...
}

This will ensure that the correct translation is loaded and displayed.

Could be, but then why does the other string in the file (which also does not have a domain specified) get translated?

(In reply to Nate Graham from comment #3)
> Could be, but then why does the other string in the file (which also does
> not have a domain specified) get translated?

You mean     

title: i18n("Shortcuts")

?

That's not being used at all, change it to blo blo blo and it will still say desktop shortcuts

I can confirm that 

            Kirigami.FormData.label: i18ndc("plasma_shell_org.kde.plasma.desktop", "@action:button set keyboard shortcut for", "Activate widget as if clicked:")


Fixes it, i have no idea what Kirigami.LocaleHelper or KCM.ConfigModule do