Now I'm having trouble logging in to GMail: that link set a cookie saving my language as Japanese! D:
edit: Found the "language" option on Google sign-in -- it's the scroller in the bottom right labelled "日本語". There's a small blue logo next to it that looks like the UN flag. Hope this helped someone.
edit #2: It's also in the URL, replace /intl/jp/ with /intl/en/ (for English)
I think users without prior IME experience may have a hard time image what is the purpose of sync or what does it do. It's a user trained Bayesian db where you can input your most used words/combinations very fast.
It's like auto-complete in IMEs or Google Suggest where you can get a list of "hints" ordered by popularity.
Baidu IME does the same, except, according to OP:
> ... sends users’ information even when the function is turned off.