Even wired speedos can sometimes be affected by the ignition system. There is a lot of discussion here regarding speedometers and interference. Use the Google Custom Search, type in 'speedometer' and you'll have plenty to rear.
As for instructions that have been translated into English, you never know what you'll see/read. The booklet that comes with the Bell speedo shows the magnet/sensor mounted on the left front fork but the instructions say to mount it on the right side