* Added Blockly.ScratchMsgs

* Added script to create combined locales `Blockly.ScratchMsgs.locales`
* Regenerated en.json after changing single quoted strings in messages.js to double quoted strings.
* Include `create_scratch_msgs.js` in `npm run translate`
locale=<locale-string>` to the URL.
* Added assertions so translate fails if `messages.js` has incorrect strings, or if any of the downloaded locale json files have errors in the translations.
* Playground will load different locales.

i18n/create_scratch_msgs.js

@@ -0,0 +1,64 @@
+const fs = require('fs');
+const path = require('path');
+const glob = require('glob');
+const assert = require('assert');
+
+// Globals
+const PATH_INPUT = path.resolve(__dirname, '../msg/json/*.json');
+const PATH_OUTPUT = path.resolve(__dirname, '../msg');
+
+let en = fs.readFileSync(path.resolve(__dirname, '../msg/json/en.json'));
+en = JSON.parse(en);
+const enKeys = Object.keys(en).sort().toString();
+
+// Check that translation is valid:
+// elt: array [key, translation] from <locale>.json
+// - messages with placeholders have the same number of placeholders
+const validatePlaceholders = function (elt) {
+    const re = /(%\d)/g;
+    const [key, translation] = elt;
+    const placeholdersCount = en[key].match(re) ? en[key].match(re).length : 0;
+    if (placeholdersCount > 0) {
+      const tMatch = translation.match(re);
+      const enMatch = en[key].match(re);
+      assert.notStrictEqual(tMatch, null, `${key} is missing a placeholder: ${translation}`);
+      assert.strictEqual(
+          tMatch.sort().toString(),
+          enMatch.sort().toString(),
+          `${key} is missing or has duplicate placeholders: ${translation}`
+      );
+    }
+};
+
+const validate = function (json, name) {
+    // this is a little stricter than we need - it would be harmless if the translation had extra keys
+    assert.strictEqual(Object.keys(json).sort().toString(), enKeys, `${name}: Locale json keys do not match en.json`);
+    Object.entries(json).forEach(validatePlaceholders);
+};
+
+let file = '';
+file += '// This file was automatically generated.  Do not modify.\n';
+file += '\n';
+file += '\'use strict\';\n';
+file += '\n';
+file += `goog.provide(\'Blockly.ScratchMsgs.locales\');\n`;
+file += `goog.require(\'Blockly.ScratchMsgs\');\n`;
+file += '\n';
+
+let files = glob.sync(PATH_INPUT);
+files.forEach(function (uri) {
+    const name = path.parse(uri).name;
+    if (name !== 'qqq' && name !== 'synonyms') {
+      let body = fs.readFileSync(uri);
+      // Convert file body into an object (let this throw if invalid JSON)
+      body = JSON.parse(body);
+      validate(body, name);
+      file += '\n';
+      file += `Blockly.ScratchMsgs.locales["${name}"] =\n`;
+      file += JSON.stringify(body, null, 4);
+      file += ';\n';
+    }
+});
+
+// write combined file
+fs.writeFileSync(`${PATH_OUTPUT}/scratch_msgs.js`, file);

i18n/js_to_json.js

@@ -1,6 +1,7 @@
 const es = require('event-stream');
 const fs = require('fs');
 const path = require('path');
+const assert = require('assert');
 
 // Storage object
 const storage = {};
@@ -12,6 +13,7 @@ const PATH_OUTPUT = path.resolve(__dirname, '../msg/json/en.json');
 // Match function
 const match = function (str) {
     if (str.indexOf('Blockly.Msg.') !== 0) return false;
+    assert.notStrictEqual(str.indexOf("';"), str.length - 2, `[${str}] uses single quoted string, should use double quotes.`);
     if (str.indexOf('";') !== str.length - 2) return false;
     return true;
 }

i18n/json_to_js.js

@@ -11,7 +11,8 @@ const CONCURRENCY_LIMIT = 4;
 // Processing task
 const work = function (uri, callback) {
     fs.readFile(uri, function (err, body) {
-        if (err) return callback(err);
+      const name = path.parse(uri).name;
+      if (err) return callback(err);
 
         // Convert file body into an object (let this throw if invalid JSON)
         body = JSON.parse(body);
@@ -22,7 +23,7 @@ const work = function (uri, callback) {
         file += '\n';
         file += '\'use strict\';\n';
         file += '\n';
-        file += 'goog.provide(\'Blockly.Msg.en\');\n';
+        file += `goog.provide(\'Blockly.Msg.${name}\');\n`;
         file += 'goog.require(\'Blockly.Msg\');\n';
         file += '\n';
 
@@ -32,7 +33,6 @@ const work = function (uri, callback) {
         }
 
         // Write file to disk
-        const name = path.parse(uri).name;
         fs.writeFile(`${PATH_OUTPUT}/${name}.js`, file, callback);
     });
 };