Even I am facing the same problem. I bought the parts in japan and assembled it but the OS is in English. While installing drivers
the installer was in Japanese. Also my keyboard layout is japanese.
Now the programs which I had in English before are getting installed in Japanese when I am doing the installation. I manually changed the language of some programs but I can't find the option in other as they are all in Japanese. I thought it was taking the location from internet and installed without the internet but they still got installed in Japanese.
How do I stop this? Some help would be really appreciated.