improvement(i18n): extract core UI texts to external file (#1708)

packages/netlify-cms-core/package.json

@@ -38,6 +38,7 @@
     "netlify-cms-lib-auth": "^2.0.4",
     "netlify-cms-lib-util": "^2.1.0",
     "netlify-cms-ui-default": "^2.0.6",
+    "node-polyglot": "^2.3.0",
     "prop-types": "^15.5.10",
     "react": "^16.4.1",
     "react-dnd": "^2.5.4",
@@ -49,6 +50,7 @@
     "react-immutable-proptypes": "^2.1.0",
     "react-is": "16.3.1",
     "react-modal": "^3.1.5",
+    "react-polyglot": "^0.2.6",
     "react-redux": "^4.4.0",
     "react-router-dom": "^4.2.2",
     "react-router-redux": "^5.0.0-alpha.8",