Локаль можно задать на этапе сборки

lib/purchase_success.dart

@@ -22,12 +22,12 @@ class PurchaseSuccessScreenState<T> extends BaseState<PurchaseSuccessScreen> {
   String sum;
   String username;
 
-  @override getMenuButtons(BuildContext context) {
+  @override getMenuButtons() {
     return <Widget>[getFaqButton(), getLogoutButton()];
   }
 
   @override String getTitle() {
-    return Strings.of(context).carryingPurchase();
+    return StringsLocalization.carryingPurchase();
   }
 
   @override String getHint() {
@@ -36,10 +36,10 @@ class PurchaseSuccessScreenState<T> extends BaseState<PurchaseSuccessScreen> {
 
   @override Widget getScreenContent() {
     return new Column(children: <Widget>[
-      getValueWithDescription(Strings.of(context).buyer(), username),
+      getValueWithDescription(StringsLocalization.buyer(), username),
       getSuccessMessage(),
       new Expanded(child: new Center()),
-      buildButton(getScreenMargins(74.0), getScanButton())
+      wrapButton(getScreenMargins(74.0), getScanButton())
     ]);
   }
 
@@ -49,8 +49,8 @@ class PurchaseSuccessScreenState<T> extends BaseState<PurchaseSuccessScreen> {
   }
 
   getScanButton() {
-    String title = Strings.of(context).scan();
-    return buildRaisedButton(context, title, () => startScanner(context));
+    String title = StringsLocalization.scan();
+    return buildRaisedButton(title, () => startScanner(context));
   }
 
   getSuccessMessage() {
@@ -61,7 +61,7 @@ class PurchaseSuccessScreenState<T> extends BaseState<PurchaseSuccessScreen> {
   }
 
   getMessageTitle() {
-    return Strings.of(context).purchaseCompleted(sum);
+    return StringsLocalization.purchaseCompleted(sum);
   }
 
 }