I was able to do this by doing the following:
- Created a new Sticker Pack Application
- Creating an InfoPlist.strings file (containing only the CFBundleDisplayName value, saving it into a localized directory (I used es.lproj for my test)
- Create a second InfoPlist.strings file, saving it en.lproj
- Added that file to my project, being sure to add it to the extension target (initially I had mistakenly added it to the parent iOS app target, which didn't work as expected).
- Built/ran on a device that was running in Spanish
So now I've got a sticker pack with Spanish and English localized Info.plist strings files. The name of the app in the Messages app drawer is showing up correctly as I switch between languages.
I would double check that you've got the files in the built .appex correctly where you expect them.