First of all, you can do
export LANG=C beore running a command to get the output in english.
The reason you can't install the package
rTorrent is that the name of the package is rtorrent.
You can search for a package using the command
apt-cache search rtorrent, which would show you the following:
$ apt-cache search rtorrent
qbittorrent-nox - bittorrent client based on libtorrent-rasterbar (without X support)
rtgui - Web based front-end for rTorrent
rtorrent - ncurses BitTorrent client based on LibTorrent from rakshasa
rtpg-www - web based front end for rTorrent
You can install rtorrent by running
sudo apt-get install rtorrent (note the lower case).
As per your update, you have already installed rtorrent, and can start it by running
rtorrent in a terminal. It additionally seems that you have missing dependencies. Please try what the output suggests;
sudo apt-get -f install. Usually this will install the needed package.