The only implementation that I found (for a swift package) that works like this is (using the new String Catalogs; untested with legacy string files)
let key:String.LocalizationValue = String.LocalizationValue(stringLiteral: "<localization key>")
return String(localized: key, table: "<String Catalog Table Name>", bundle: Bundle.module)
I have this implemented and working in one of my SPM projects, but you'll need to change Bundle.module to Bundle.main, or just leave it blank, for it to maybe work in an App (I haven't tested it).