In your view, you should just use
<%= f.input :gender, collection: [:male, :female] %>
In your yaml file, you should have
simple_form:
options:
user: (or defaults:)
gender:
male: Male
female: Female
This should automatically translate into
<option value="male">
Male
</option>
It is the values that will go back to the controller, not the translated words.
If not, there is something wrong...