Angelina so at least in germany it is not possible for the parents to do something like that
That's actually wrong.
A psychiatric clinic is for dealing with mentally ill patients and the criteria for a someone being admitted to a locked down ward where he / she cannot leave is based on the risk of harming oneself or another person.
This is completely different from guardianship. Like parents are the legal guardians of their u der age children, a court can appoint a guardian for a person over the legal age of maturity (18 in Germany) and whether or not strong measures taken by a guardian are acceptable is always a case by case decision.
You could for example argue that normally putting someone in a belt should be illegal, but this again would be determined based on an individual case. If a guardian can, for example, demonstrate he havs exhausted less restrictive measures and was unable to achieve the necessary control to actually protect the person under Guardiansip, a court might well rule that use of a cb was actually justified.