Anthony,
If you need your site localized in 15 languages and, at the same time, have it up and running as fast as possible, I strongly suggest not to implement a fully ML solution. Any kind of ML solution will require the participation of people speaking the languages you need so that they fill the actual dynamic content throughout the site, and that means a lot of work and coordination. IMHO, a ML Portal is an appropriate solution for a site having 2, 3 or up to, say 5 languages, and that is a lot.
Sebastian is correct in pointing out that the Language Packs you may install on your site will only translate static text, related to the site's administration and operation. This means that, for example, the registration page, login page and other user's maintenance pages will be spoken in each language you install. And, more important, the forum and blog (or other complex modules) will have all their functional messages and instructions spoken in the client's language which is fundamental if you want those users to participate seamlessly.
So, to make it short, my advice would be:
1. Install one Language Pack for each language you want to support. Try to find the most complete LP available, not only at the DotNetNuke.com's repository, but also browsing the Internet.
2. There is an awesome tool available from Google, called Google Translate. You may insert a code snippet in all your site's pages. This tool performs an acceptable work with many languages, not all, but (IMHO and AFAIK) is much more powerful than other free available tools (and paid too), such as Altavista's, Yahoo!'s, BabelFish's and others. You can see it working at our site, http://www.DNNCastellano.com, where we use it to translate dynamic content from Spanish to English.
One note that should be taken regarding any automatic translation tool: It will never be perfect ;-) ...well, sometime in the future, maybe... :-)
HTH.
Saludos,
Alejandro.