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The Strings files are text files with the .Strings extension, containing pairs of keys and values, that can be accessed and rendered in the UI.
These files reside in separate folders, e.g. en.lproj, de.lproj. But in the project tree they are grouped by the strings file name. AppCode shows the files the same way as Xcode. To learn more, see the Internationalize Your App section of the Xcode documentation.