Just open the control panel and go to add languages, add english as an option and move it to the top.
After that, go to options (on english) and set it as the windows display language.
If it tells you to download english first, then do that (just click there and it'll install itself). You may need to restart, but once it's done go back to the same add language screen and press advanced settings.
There you have an option to apply to welcome screen and other sections.
Finally, choose the option below advanced settings (change date time and number formats), go to the administrative tab, and change the unicode settings to United States.
With that you should have English as the main language.
If you want only English on that machine, you will need to reformat (lose everything), so after backing up your files and making sure you have all your software product keys, just follow the instructions at
http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media to make a new install medium using the english language version (you can pick it from the options). You should be able to use your current key just fine.
If you want to upgrade to Windows 10, the methods will be identical.