Internationalization (#131)

Replaced hardcoded English strings with locale-based variables based on the [easy_localization](https://pub.dev/packages/easy_localization) Flutter plugin.

pubspec.lock

@@ -141,6 +141,20 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.5.4"
+  easy_localization:
+    dependency: "direct main"
+    description:
+      name: easy_localization
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "3.0.1"
+  easy_logger:
+    dependency: transitive
+    description:
+      name: easy_logger
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.0.2"
   fake_async:
     dependency: transitive
     description:
@@ -216,6 +230,11 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "6.0.0"
+  flutter_localizations:
+    dependency: transitive
+    description: flutter
+    source: sdk
+    version: "0.0.0"
   flutter_plugin_android_lifecycle:
     dependency: transitive
     description:
@@ -282,6 +301,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.3.1"
+  intl:
+    dependency: transitive
+    description:
+      name: intl
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.17.0"
   js:
     dependency: transitive
     description: