Nataraj,
I know you don't want to redeploy you application each time a string changes, but calling an external service can also complicate things. I say this from experience as a group at my company did a very large (200K line, 400 screen) GWT app last year which had to work in many languages. They did not use the GWT i18n tools because the company wanted to use a service like you describe. However, the result was a much slower application because various parts of the GWT code had to wait for the i18n files to be loaded from the company's (very slow) i18n service before it could render anything on a given page or screen. In hindsight, they wish they had used the GWT i18n facilities since such strings rarely are ever changed in production.
Hope that helps.
Sincerely,
Joseph