Check the simple-form documentation regarding i18n: https://github.com/plataformatec/simple_form#i18n
It is pretty well described, and goes into great detail.
So for instance (from the documentation) :
en:
simple_form:
labels:
defaults:
username: 'User name'
password: 'Password'
new:
username: 'Choose a user name'
hints:
defaults:
username: 'User name to sign in.'
password: 'No special characters, please.'
placeholders:
defaults:
username: 'Your username'
password: '****'
This specifies the defaults for labels. You should check the documentation, because there are a lot more options.
Secondly, you can also do it explicitly, like so
= f.input :car, label: I18n.t('my_car_label'), prompt: I18n.t('my-car-prompt'), placeholder: I18n.t('my-placeholder')