Autocorrect in Safari- OSX Yosemite comes with a default autocorrect feature, switched on. And so does Safari. In some combinations of Safari and OSX Yosemite versions the program autocompletes words as you type them on thebrowser, not only in forms, but also in Tweetdeck, Mail, Skype etc. This happens to Fluenz (not always) as well. It's a feature of Safari. We can't control it on our end. It's like turning autocorrect off on your phone. You have to do it on your end.
To turn it off overall for all of the OSX: Apple Menu > System Preferences > Keyboard > Text Turn Off: Correct Spelling Automatically
To turn it off in Safari: Open your browser. In the top menu click EDIT > SPELLING AND GRAMMAR
Make sure "Correct Spelling Automatically" is not switched on.
PS Something similar can happen if you have your translator on. Be sure that your translator is off, otherwise the non-English words in the program will be translated by your OS.