GitLab CI: check yarn i18n

.gitlab-ci.yml

@@ -62,6 +62,12 @@ build:
     paths:
      - soapbox.zip
 
+i18n:
+  stage: test
+  script:
+    - yarn i18n
+    - git diff --quiet || (echo "Locale files are out of date. Please run `yarn i18n`" && exit 1)
+
 docs-deploy:
   stage: deploy
   image: alpine:latest