I18n: Improve tooling (#56731)

* Delete en-US.json before extracting strings so it's always up to date

* Add 'comment' to message jsons

* Enable Pseudo-locale option only in dev mode

* don't load english messages

* fix package.json

* fix

public/locales/fr-FR/grafana.json

@@ -1,4 +1,5 @@
 {
+  "_comment": "Do not manually edit this file. Translations must be made in Crowdin which will sync them back into this file.",
   "common": {
     "locale": {
       "default": "",