Локаль можно задать на этапе сборки

lib/main.dart

@@ -2,8 +2,23 @@ import 'package:flutter/material.dart';
 import 'splash.dart';
 import 'consts.dart';
 import 'strings.dart';
+import 'common.dart';
 import 'dart:async';
 
+class StringsLocalizationDelegate extends LocalizationsDelegate<StringsLocalization> {
+
+  @override
+  Future<StringsLocalization> load(Locale locale) async {
+    return StringsLocalization.load(await platform.invokeMethod("getLocale"));
+  }
+
+  @override
+  bool shouldReload(LocalizationsDelegate<StringsLocalization> old) {
+    return false;
+  }
+
+}
+
 /// Точка входа в приложение.
 void main() {
   runApp(new Checker());
@@ -18,7 +33,12 @@ class CheckerState extends State<Checker> {
   @override Widget build(BuildContext context) {
     return new MaterialApp(
       title: appName,
-      home: new SplashScreen()
+      home: new SplashScreen(),
+      localizationsDelegates: getLocalizationsDelegate()
     );
   }
+
+  getLocalizationsDelegate() {
+    return <StringsLocalizationDelegate>[new StringsLocalizationDelegate()];
+  }
 }