I faced the same issue and I was able to resolve it because I noticed that the InfoPlist.strings wasn’t member of any target.
So setting the Target Membership on the file (which puts it into the Copy Bundle Resources build phase) fixed it.
And for anyone googling here: https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/AboutInformationPropertyListFiles.html
Scroll down to “Localizing Property List Values”