Can't localise my plug-in

Hi,

I'm ready to ship a new plug-in for macOS, but the German localisation doesn't work. My plug-in uses the strings from en.lproj/Localizable.strings but not the one in de.lproj/Localizable.strings in my .coplugin bundle.
Is this a (known) bug?

Xcode 10.1 (10B61)
Capture One Pro 12.0 Build 12.0.0.270 (02a5fb5)
macOS 10.14

Best regards

Uwe