Because "had worked" here would be referring to some event that was one further step into the past than Jane's dad working in Germany. Think of it in "steps" (we call it the sequence of tenses):
pluperfect 'had worked' <- simple past 'worked' <- present 'works'
If the sentence was "Well, he had worked on a project with some German engineers when he decided to move to France", that's fine, because there's another event before it in the sequence. As it is, you have to use "worked" as it's the only event in the past that's mentioned.