I think the trouble with using ProjectDisplayedTitle is that it’s an FText, not FString, which the build tools don’t properly understand so you may still get an incorrect value.
For now the simplest is probably to add a [Game]\Config\UWP\UWPEngine.ini in which you set Package.Properties.DisplayName to a literal value of your own choosing. Alternatively for a one-off you can hand-edit the appxmanifest, and then re-run makeappx from the command line.
Really though this field should have a value something like ms-resource:ProjectName where ProjectName is a key in a localized resw file (that gets compiled into the resources.pri file which ultimately goes into the appx). The process for doing this properly is kind of half-baked at present - resw files are actually being generated to an intermediate directory, but there’s no current mechanism to the [Game]\Intermediate\Deploy directory, but there’s no mechanism to put localized source content into them. I’m experimenting with some small changes as a stopgap improvement for this. Will update here if they work out.