Courts in your state (CO) are not bound to always enforce foreign court judgments, but generally will if the judgment is from a court in a nation which generally extends the same courtesy to U.S. courts and where the CO court is confident that the foreign court provided due process. German courts would fall under those rubrics, so it is very likely that if they were to sue you in Germany and win, that they could then file a legal action in CO and get a CO court to enforce it the way they would a CO judgment. It's a multi-step process, and can be slow and expensive, so they might not do this, but legally, they have this option.