Add a mechanism for customizing app strings

Tools/SwiftGen/Templates/Strings/structured-swift5-element.stencil

@@ -78,6 +78,11 @@ extension {{enumName}} {
     // Use preferredLocalizations to get a language that is in the bundle and the user's preferred list of languages.
     let languages = Bundle.overrideLocalizations ?? Bundle.app.preferredLocalizations
 
+    // Letro: customize strings
+    if let translation = trLetro(key, args, languages: languages) {
+      return translation
+    }
+
     for language in languages {
       if let translation = trIn(language, table, key, args) {
         return translation
@@ -86,6 +91,19 @@ extension {{enumName}} {
     return Bundle.app.developmentLocalization.flatMap { trIn($0, table, key, args) } ?? key
   }
 
+  private static func trLetro(_ key: String, _ args: [CVarArg], languages: [String]) -> String? {
+    // Letro: customize strings
+    // Fall back to English Letro overrides
+    for language in languages + ["en"] {
+      guard let bundle = Bundle.lprojBundle(for: language) else { continue }
+      let format = NSLocalizedString(key, tableName: "Letro", bundle: bundle, comment: "")
+      let translation = String(format: format, locale: Locale(identifier: language), arguments: args)
+      guard translation != key else { continue }
+      return translation
+    }
+    return nil
+  }
+
   private static func trIn(_ language: String, _ table: String, _ key: String, _ args: CVarArg...) -> String? {
     guard let bundle = Bundle.lprojBundle(for: language) else { return nil }
     let format = NSLocalizedString(key, tableName: table, bundle: bundle, comment: "")