From what I can tell from looking at the UserServiceImpl.setUserActivationKey method, the "reset password" email will always be sent out in the system / context default locale.
However, since we are sending this email out to a specific, existing user in the system, we should be able to determine their preferred locale. From OpenmrsConstants, we see that the "USER_PROPERTY_DEFAULT_LOCALE" constant assumes a user property named "defaultLocale".
This property is used by emrapi and the referenceapplication to set/get this and use it in the application. This email functionality should do the same.