OpenNLP Coreference Resolution (German)
From an older version of the OpenNLP README:


  Training the Tools
  
  There are training tools for all components expect the coref component. Please consult the help message of the tool and the javadoc to figure out how to train the tools.
  
  The tutorials in our wiki might also be helpful.
  
  The following modules currently support training via the WordFreak opennlp.plugin v1.4 (http://wordfreak.sourceforge.net/plugins.html).
  
  
  coreference: org.annotation.opennlp.OpenNlpCoreferenceAnnotator (use opennlp 1.4.3 for training, models are compatible)
  
  
  Note: In order to train a model you need all the training data. There is not currently a mechanism to update the models distributed with the project with additional data.


As you can see, OpenNLP does not provide training tools for the coreference component. However, it seems at one point it was possible to train new models for OpenNLP's coref component using the third-party WordFreak plugin... however, it hasn't been updated in over a decade, so your mileage may vary.