diff --git a/Dockerfile b/Dockerfile
index c57e2dba79..a5ea4505a0 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -33,7 +33,12 @@ COPY test/chai-as-promised.js /grist/test/chai-as-promised.js
 COPY app /grist/app
 COPY stubs /grist/stubs
 COPY buildtools /grist/buildtools
+# Copy locales files early. During build process they are validated.
+COPY static/locales /grist/static/locales
 RUN yarn run build:prod
+# We don't need them anymore, they will by copied to the final image.
+RUN rm -rf /grist/static/locales
+
 
 # Prepare material for optional pyodide sandbox
 COPY sandbox/pyodide /grist/sandbox/pyodide
diff --git a/buildtools/sanitize_translations.js b/buildtools/sanitize_translations.js
index 874c7392f3..016f7bf0c1 100644
--- a/buildtools/sanitize_translations.js
+++ b/buildtools/sanitize_translations.js
@@ -14,6 +14,12 @@ function handleSanitizeAttribute(node) {
   node.setAttribute('target', '_blank');
 }
 
+// If the the first arg is test, do the self test.
+if (process.argv[2] === "test") {
+  selfTest();
+  process.exit(0);
+}
+
 const directoryPath = readDirectoryPath();
 
 const fileStream = fs.readdirSync(directoryPath)
@@ -25,38 +31,37 @@ const fileStream = fs.readdirSync(directoryPath)
                      // Read the contents and put it into an array [path, json]
                      .map((file) => [file, JSON.parse(fs.readFileSync(file, "utf8"))]);
 
-console.debug(`Found ${fileStream.length} files to sanitize`);
-
 const sanitized = fileStream.map(([file, json]) => {
-  return [file, json, sanitizedJson(json)];
+  return [file, json, invalidValues(json)];
 });
 
-const onlyDifferent = sanitized.filter(([file, json, sanitizedJson]) => {
-  return JSON.stringify(json) !== JSON.stringify(sanitizedJson);
-});
-
-console.debug(`Found ${onlyDifferent.length} files that need sanitizing`);
-
-// Write the sanitized json back to the files
-onlyDifferent.forEach(([file, json, sanitizedJson]) => {
-  console.info(`Sanitizing ${file}`);
-  fs.writeFileSync(file, JSON.stringify(sanitizedJson, null, 4) + "\n");
+const onlyDifferent = sanitized.filter(([file, json, invalidKeys]) => {
+  return invalidKeys.length > 0;
 });
 
-console.info("Sanitization complete");
+if (onlyDifferent.length > 0) {
+  console.error("The following files contain invalid values:");
+  onlyDifferent.forEach(([file, json, invalidKeys]) => {
+    console.error(`File: ${file}`);
+    console.error(`Values: ${invalidKeys.join(", ")}`);
+  });
+  process.exit(1);
+}
 
-function sanitizedJson(json) {
+function invalidValues(json) {
   // This is recursive function as some keys can be objects themselves, but all values are either
   // strings or objects.
-  return Object.keys(json).reduce((acc, key) => {
-    const value = json[key];
+  return Object.values(json).reduce((acc, value) => {
     if (typeof value === "string") {
-      acc[key] = purify(value);
+      const sanitized = purify(value);
+      if (value !== sanitized) {
+        acc.push(value);
+      }
     } else if (typeof value === "object") {
-      acc[key] = sanitizedJson(value);
+      acc.push(...invalidValues(value));
     }
     return acc;
-  }, {});
+  }, []);
 }
 
 
@@ -72,5 +77,49 @@ function readDirectoryPath() {
 
 function purify(inputString) {
   // This removes any html tags from the string
-  return DOMPurify.sanitize(inputString);
+  return DOMPurify.sanitize(inputString, { ALLOWED_TAGS: [] });
 }
+
+function selfTest() {
+  const okDir = createTmpDir();
+  const okFile = path.join(okDir, "ok.json");
+  fs.writeFileSync(okFile, JSON.stringify({ "key": "value" }));
+
+  const badDir = createTmpDir();
+  const badFile = path.join(badDir, "bad.json");
+  fs.writeFileSync(badFile, JSON.stringify({ "key": "<script>alert('xss')</script>" }));
+
+  // Run this script in the okDir, it should pass (return value 0)
+  const okResult = exitCode(`node ${__filename} ${okDir}`);
+  if (okResult !== 0) {
+    console.error("Self test failed, expected 0 for okDir");
+    process.exit(1);
+  }
+
+  // Run this script in the badDir, it should fail (return value 1)
+  const badResult = exitCode(`node ${__filename} ${badDir}`);
+  if (badResult !== 1) {
+    console.error("Self test failed, expected 1 for badDir");
+    process.exit(1);
+  }
+
+  console.log("Self test passed");
+
+  function createTmpDir() {
+    const os = require('os');
+    const tmpDir = os.tmpdir();
+    const prefix = path.join(tmpDir, 'tmp-folder-');
+    const tmpFolderPath = fs.mkdtempSync(prefix);
+    return tmpFolderPath;
+  }
+
+  function exitCode(args) {
+    const {execSync} = require('child_process');
+    try {
+      execSync(args); // will throw if exit code is not 0
+      return 0;
+    } catch (e) {
+      return 1;
+    }
+  }
+}
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