It depends on the training. For some trainings there are no requirements for particular educational or work related background or previous training. For other trainings, it is a requirement to have a certain educational background, certain experience or to have completed other trainings.
It is always a requirement that the participants are able to understand and express themselves fluently in the language of the training, and that they are prepared to participate actively in the training (e.g. express their opinions in discussions, share experiences with the group, participate in group work, role plays etc.). For training of trainers, the participants are also expected to facilitate a session during the workshop.
Requirements for each training can be found in the description of the training.