You can create new template files for your second locale and simply use Twig include to only maintain one template for all of your locales.
projekte.html:
{% include 'projects.html' %}
Use a locale specific subfolder (craft/templates/de/) to separate the german templates from your main / first locale templates (see the Template Localization article in the Docs on how this works).
Another way to handle localized URIs is to not use "URI to template matching" (pt. 6 in the Routing article) but configure an entry that loads your template instead (pt. 4).
Create a single section with "projects.html" for the template that should get loaded if the single's URL is requested. The template's filename is irrelevant for the URI matching and you can set the entry's slug to whatever you want in both of your locales.
Before creating many single sections now, also consider using a "Pages" structure section instead.