Just don't worry. The most important rule in a project like this is "be as careful as you can," but I and probably three other people will check every commit by new German team members for obvious pitfalls, and then there's the automatic nightly test. It is almost impossible that you break something crucial as long as you don't touch the Java code.
Even if a bad XML rule gets through the defenses, it will usually not harm the system as a whole. The worst thing that can happen is that LT will produce some weird error messages. It is doing that all the time every now and then anyway (all grammar checkers have this problem).