This is a quite strange question: why some parts of the android studio wesite aren't written in English but in Japanese?
and why Android Studio on startup tells me that there is an update available but if I visit the official website and start the download it is downloaded a less recent version than the one installed on my computer?
For the first question, it's probably because you're on the Japanese website. The English one / official one is here : http://developer.android.com/sdk/index.html
For the version, it's not important. With the new intellij/android studio, update come through the software.