add translation strings and add test case (#9294)

api4/user_test.go

@@ -69,6 +69,11 @@ func TestCreateUser(t *testing.T) {
 	CheckErrorMessage(t, resp, "model.user.is_valid.email.app_error")
 	CheckBadRequestStatus(t, resp)
 
+	ruser.Username = "testinvalid+++"
+	_, resp = Client.CreateUser(ruser)
+	CheckErrorMessage(t, resp, "model.user.is_valid.username.app_error")
+	CheckBadRequestStatus(t, resp)
+
 	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.TeamSettings.EnableOpenServer = false })
 	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.TeamSettings.EnableUserCreation = false })