internationalization??? did you read the question?
what you want to do is not easily done using struts since it provides only basic frontend support. the rest is your creativity. the closest thing that you will find useful is tiles, which allows for you to have SOME UI flexibility. still with tiles, you are limited to pre-defined configurations (that is pre-defined templates) and will not be able to go more fine-grained.
your better approach for fine-grained frontend control is to use stylesheet controls. my favorite one is iht.com where you can dynamically resize font and column format. there are much more advanced controls, but you'll need to explore CSS options.
another way i can think of is to generate the CSS styles based on user-selected options. pass the font size, color, and other things into the
jsp which contains all the CSS style defs where the definitions will be partially fixed and partially dynamic: ex. pageTitle{border:<c
ut value="${userOptions.borderColor}"/>;}