This is possible. You can do it by hand right now, by installing different languages of Windows Live Writer and copying the files from the C:\Program Files\Windows Live\Writer\Dictionaries to another location, then installing the other language you want and copying the files back into that folder. If you poke around the internet you might be able to find download links to all the dictionaries, they are all hosted on download.microsoft.com but I don't have access to the URLs right now.
We hope to improve this in the future and make it possible to download dictionaries from inside WLW. Thanks for the feedback.