It's going to be a lot of work, probably more than if the application was localized to begin with, because now you have to remove/edit things instead of just creating them.
Even if there is an automated tool to help out, you'll want to go through each form one by one to make sure the formatting won't break for localized strings (some languages are generally longer than others; i.e., German vs. English). This should be easier for a web forms application, but you didn't specify the application type, so I'll try to give a medium-neutral answer here.
Have a look at my answer to this question for my strategy on distributing the localized strings through your application.
The biggest issue I've seen are localized strings where the language syntax is important and code is injecting values into the strings programmatically. Try to avoid these as they will cause you nothing but pain. I remember a question on here about localizing a string such as "x days, y hours, z minutes" -- or something to that effect.
The strategy I use in most of these situations is to find a way to display the same data in a locale-neutral way, if at all possible. For example, instead of having a single label with the text "45 minutes" refactor it to separate the label from the value as "Minutes:" and "45". Things like that. Refactoring a UI is as much an art as a science, so it's difficult to give a specific answer without knowing what you're dealing with, or the business domain, or use cases, etc.