Yep. If you want to get fancy, create templates/german and templates/german/local - put your original templates in the upper-level directory, then you can use the version control of the online template manager for making changes if you want. Remember to put a .tplinfo file in german - to call up the editor controls.
You can also let the user select which set of templates to use. In user_profile_display.html, add something like this (code from my site for switching between a graphics-heavy template and a barebones one; in your case, you'd use "german" instead of "bare"):
<option value="default"<%if user_template eq default%> selected<%endif%>>Default</option>
<option value="bare"<%if user_template eq bare%> selected<%endif%>>Bare</option>
Once you've created your german directory (or directories), I think you should be able to modify its error messages by going into the admin console, then Templates, then Language Vars, then select "german" from the dropdown menu.