It is possible to customise the reset password email content on Reset Password Email tab. Follow steps below to find it:
Open System Configuration tab
Expand Login Settings
Click on Reset Password Email

Reset Password Email window
Reset Password Email window contains language tabs according to active-languages configured in webapp.yaml and System language tab is selected by default.
In order to create custom templete of selected language:
Switch to language tab
Update Email Subject
Update Email Body
Save changes
Note! Email Sender field is not editable. Information is taken from from Mail Properties settings.
Language of the Reset Password mail to be received by the user will be:
specific language set per user on any first site accessible for the user;
site default language on any first site accessible for the user - if no specific language set;
system default language - if the user is system admin, or no sites found, or user is expired.