The resources for the form are stored in separate files, with one file per language. The resource files are named form-resources-en.xml and form-resources-es.xml. (Additional files would be added to support additional languages.)
To internationalize the form, you will need to edit those files. To facilitate your job, you should use a tool that makes it easy to compare files, such as "WinMerge":http://winmerge.org/. This tool will provide you a view that looks as follows:
In most cases, you will be editing the resources for a language such as Spanish while comparing the resource you're editing to the original in English. During this process, you'll want to make sure that you're only changing the "text" inside the elements, leaving the tags unchanged. Don't change the indentation or order in a file for a language, as this will make it harder to compare the resources for this language to the original resources in English.
Once you have modified the file, you just need to save the file and reload the form in the browser to see your changes. You might want to use Firefox to test your changes, as with Firefox, when you do a reload (ctrl-r or F5), the form will be reloaded from the server but then put back in the same state as it was before. This means that if you had switched to a language other than English, or had opened some sections, you won't need to do that again.