The "Missing" Localization Glossary for Apple terminology in a dozen languages for common terms like Settings, Preferences, Copy & Paste, etc. Since Apple does not provide one, this is a great resource.
Full list of the languages and locale codes used by Apple for iOS and Mac apps. These are used to name localization folders (e.g. fr.lproj).
A thorough tutorial with pictures to walk through localizing an iOS/Mac app for the first time using Xcode. Steps include:
1. Preparing your iOS app for localization in Xcode 12.x
2. Extract your hard-coded strings to NSLocalizedString()
3. Use Xcode's import/export localization features