Does a WLAN Repeater knows to repeat any signal data (whether it is encrypted or not),
or are there requirenments, which this repeater has to fulfill, like knowing about encryption methods, protocols etc. ?
A true repeater needs almost no information, it literally just repeats incoming traffic the other way. They certainly don't need to look inside packets and should be totally unaware of whether a packet even contains IP data let alone any other protocol or encryption data, in fact could be anything that adheres to wireless ethernet standards - you could make up your own protocol and it'd just repeat it.
That said some repeaters are really more like routers that do look inside packets but even they will have a dumb/fallback mode of just blindly repeating if they can't get the information they need.