First of all, if you run a Windows 7 version that doesn’t support multi-language you won’t be able to use the Multilingual User Interface (MUI) functionality to change the default language for Windows 7 welcome screen. Unfortunately, MUIs are only available in Windows 7 Ultimate and Enterprise edition.
Basically, you are able to change that only when the Keyboards and Languages tab in Region and Language setting looks like below.
To change the default language on welcome screen
Go to Administrative tab in Region and Language window, which can be launched from Control Panel, and click on Copy settings button.
You will see what the current language settings on Current user, Welcome screen, and New user account.
To make the default language setting on Welcome screen the same as your current user account, simply check the option “Welcome screen and system accounts” at the bottom.
Click OK to save the setting, and reboot the computer for the change to take effect.