Hi Kyle!
Before I go into this, let me explain one thing. Search engines operate by country, not language. So if your client has made a German language version of their site, a duplicate of their English site, then it should not just target Germany. There are many other countries and people that speak German. Rather, I would submit a sitemap to google of the german content only and let them crawl the content in German. The indexation should raise over time naturally, and if you track that subfolder in Analytics, you should be able to see what countries are landing on the German language content naturally.
If you wanted to launch a site for the German people (citizens of Deutschland), you should recommend that the content and business be developed for that country and either launched as a subfolder or their domain.de acquired. Only if the content is different because different targeting is required should you launch a site (or area) just for a country. In terms of the TLD, this is simply because if you launch a new domain and target to the country, that is a whole other site that will need link building. That's a lot of work. If you launched to target with a subfolder, you could actually target with WMT to the country Germany, but again, the content needs to be different than the main site.
So ask them what the point is, just to translate or to target Germany. That changes what you should do. But I think it's just the language, so I say let it happen, track indexation and the language subfolder in Analytics and make sure the SEs understand that it's german language content. That'll be shown to users in any country that wish their content to be in German.
Kate