Never edit core files in Magento!
Solution without changing core files
You can define custom translations in your theme, create the following file if it does not exist yet:
en_US if you use a different language)
Note that this should be a project specific theme, not
some_bought_theme/default. If you did not already, you can add a child theme next to
default and specify this one as theme in the system configuration. You don't need to copy anything from the
default directory since Magento falls back automatically, so you only need to add the files that are customized.
translate.csv you can add your custom translations. Since the translated string is "Middle Name/Initial" and it's coming from the
Mage_Customer module, you need to add this line: