In Germany, tenants are generally allowed to install a privacy screen on the balcony to provide more privacy or simply to keep the sun's rays at bay. However, there are rules to follow and the landlord has to be asked for permission.
For a really effective solution that protects against the sun and prying eyes, you need the permission of the landlord. There are a few exceptions that serve the same purpose and that may be used by the tenant without permission. Parasol and plants do not need a permit as balcony privacy screens.