The most obvious thing that springs to mind is that your labels will most likely be a different length to the English version. This could play havoc with your layout.
You could either end up with acres of white space or labels crowded in and even overflowing the space available.
You need to make your layout flexible and flow regardless of the size of the contents. Having a layout that can cope with different screen resolutions will go a long way to solving this problem.
The other big issue is right to left language support. This will again require your layout to be flexible.
It's not just the menus and dialogs that need translating. It's all the support documentation, help files, tool tips and window title captions as well. There's usually a lot more text than you realise in an application and there will always be a few strings that were added directly rather than as resources.