CE-marking, conformity with all relevant regulations.
No satutory national regulations
There are no special statutory regulations. But network operators, responsible for a safe and reliable operation of the networks, usually set requirements. Mostly it is compliance with the standards DIN VDE V 0100-551 and DIN VDE V 0100-551-1. Furthermore plug and play panels msut only be connected via a special energy plug according to VDE V 0628-1.
For the user there are obligations to register the solar-panels at the network operator as well as at the national register. Also requirements regarding the installation exist. See:
VDE Verband der Elektrotechnik Elektronik Informationstechnik e.V. - Plug and Play PV-installations
Marktstammdatenregister at Bundesnetzagentur - Registration of PV-installations
DIN VDE V 0100-551 "Low-voltage electrical installations Part 5-55: Selection and election of electrical equipment – Other equipment – Clause 551: Low-voltage generating sets"
DIN VDE V 0100-551-1 "Low-voltage electrical installations Part 5-55: Selection and election of electrical equipment – Other equipment – Clause 551: Low-voltage generating sets – Installation of generating sets for parallel operation with other energy sources including a public distribution network"
VDE V 0628-1 "Energy plug and socket outlets Part 1: Feeding into separate electric circuits"