I've read the wikipedia article on socks and cannot find information about encryption between client and proxy-server. There is only information about authentication.
Is socks5 connection a plain-text?
SOCKS5 is just a transport protocol on top of TCP/UDP but below application layer. Thus it's comparable with TCP and UDP, too. There's no inherent encryption in SOCKS, but you application has to care about this (it's not a VPN technology, but a proxy at last). If you want encryption, the protocol you speak inside of the SOCKS-channel has to provide it.