Yes, it's possible. Pretty much the same way you localize all your resources. You just need to provide a Default.png image and then enable all the languages you're interested in. The result is that you'll get a copy of the image for each language into the respective folders. Then edit (or overwrite) each image as you need. I have tested it right now and it works fine (on iOS 5 but I believe is the same for older versions). Here is a screenshot for easy reference:

EDIT: Regarding any doubts about whether or not this is a good practice, this is straight from Apple's documentation:

In addition to including the launch images at the top level of your bundle, you can also include localized versions of your launch images in your app’s language-specific project subdirectories. For more information on localizing resources in your app, see Table 6-2.