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/en-US/grafana.json

@@ -1,4 +1,5 @@
 {
+  "_comment": "Do not manually edit this file, or update these source phrases in Crowdin. The source of truth for English strings are in the code source",
   "common": {
     "locale": {
       "default": "Default",