Many of the layouts in Lockwise are constructed using Storyboards. In order to support localizing these storyboard files, make sure that the actual xml for the layout lives in the Base.lproj directory, and language-based directories such as en.lproj contain only .strings files. This can be confirmed in Interface Builder by looking at the Identity tab in the right toolbar, and confirming that the Localization section specifies Localizable Strings for each language as shown in the image.
There are many Views, Presenters, and Stores with code useful in both the Lockwise application and the CredentialProvider app extension. In service to iOS' restriction that apps and their extension run in separate processes (extension documentation here), the following code sharing strategy minimizes the amount of code required in each to keep binary sizes small and performance snappy.
We have three directories at the top level of the project -- lockbox-ios, CredentialProvider, and Shared. Each of those directories is broken down into the same set of folders. All classes in the Shared directory are included in both the Lockwise and CredentialProvider targets; the other directories' code is included only in their respective targets.
For classes that are used in both the CredentialProvider and Lockwise, the shared code lives in a Base implementation for the class (e.g. BaseDataStore, BaseAccountStore). Each target then implements its own version of the Base class, with Lockwise.app filenames simply removing the Base (e.g. DataStore, AccountStore) and CredentialProvider.appextension filenames prepending Credential (e.g. CredentialDataStore, CredentialAccountStore).
The Base sharing strategy is clunky and can be confusing when interacting with code that spans a Base- class and its target-specific implementation. Future work might see this strategy move to the recommended .framework method for code sharing.