having this talk over dinner sounds nice :-)

german law or courts operate more on common sense than the letter of the law. to be able to reject a task from your boss the task has to be unreasonable. training/mentoring is not unreasonable unless the work is dangerous and/or a trainer needs specific qualifications. running a class that teaches some generic topic like a programming language would probably be unreasonable because you could just hire an external trainer for that. the training has to be rather specific to the individual experience of the senior or some company internal knowledge to be reasonable. so this doesn't apply to the scenarios you seem to have in mind. in any case from what i read it is recommended to talk to a lawyer before refusing.

but also, if the request interferes with your ability to do your normal work then you can and should speak up and you can reasonably refuse if your boss doesn't make accommodations for that. if he doesn't then refusal is reasonable, not because training wouldn't be your job, but because your workload has increased, which is not ok.