add translation strings and add test case (#9294)

i18n/en.json

@@ -4706,6 +4706,14 @@
     "id": "model.user_access_token.is_valid.user_id.app_error",
     "translation": "Invalid user id"
   },
+  {
+    "id": "model.user.is_valid.email.app_error",
+    "translation": "Please enter a valid email address."
+  },
+  {
+    "id": "model.user.is_valid.username.app_error",
+    "translation": "Username must begin with a letter and contain between 3 and 22 characters including numbers, lowercase letters, and the symbols \".\", \"-\", and \"_\"."
+  },
   {
     "id": "model.utils.decode_json.app_error",
     "translation": "could not decode"