A. Yes. All services of this website are available for free. Paid-services for premium users might be provided in the future, but now it's all free.
A. After sign-in, you can change it from "User's menu → Settings → Language (言語)" or click here.
A. After sign-in, you can change it from "User's menu → Settings → Dictionary (使用する辞典)" or click here. A dictionary used in this web service is based on your dictionary setting, such as in a search box on a header section, on your today's brushup page, on vocabulary test results and so on.
A. Change your "My page privacy" setting to "private". After sign-in, you can change it from "User's menu → Settings → My page privacy" or click here. After you change it to private, your "My Page", "Today's Brushup" and "History of Tests" are not disclosed.