You can create your own language overrides for the strings used by the Joomla user login system.
There is the list of the language strings used for Username/Email labels:
; Component Login View
COM_USERS_LOGIN_USERNAME_LABEL="Username"
; Module Login
MOD_LOGIN_VALUE_USERNAME="Username"
; Reset Password
COM_USERS_FIELD_PASSWORD_RESET_LABEL="Email Address"
COM_USERS_RESET_REQUEST_LABEL="Please enter the email address for your account. A verification code will be sent to you.
Once you have received the verification code, you will be able to choose a new password for your account."
COM_USERS_FIELD_RESET_CONFIRM_USERNAME_LABEL="Username"