Kevin - Does Start Page (or Home page) is published in your new language.
I created a test. I added a new language DE and publish a product in a new language. It gives me a 404 error. When I publish start page in new language it started working fine.
Naveed's answer is a good thing to keep in mind. The start page has to be published in that language (there is a warning for that if you use Catalog UI).
For publishing content in new language, it is quite easy. Just write a scheduled job to iterate over content and publish them. You can use the recipe here https://leanpub.com/b/epicommercedevelopment#bundle-page-epicommercerecipes (free samples)
We have been looking into adding an additional language on to our existing Commerce site. I've done some research and even tried on a local environment and have found that while the CMS side of things is actually quite simple, when trying to bring the existing catalog over I run into issues.
The main issues that I was running into was getting a 404 page when trying to access a product page that was republished, along with its categories, with the new language. I was still able to access the current site's only language version of the product, but not the new language.
I'm not putting it beyond something I have done wrong, as I'm sure it is, but my first simple question is does anyone know of a simple way of converting/publishing a catalog with a new additional language? This seems to be one of the biggest hurdles that we will have and I kind of want to get this one answered first to see if it is something tedious of having to go through and publish everything manually or if there is a way to automate it.