This always depends on theme integration (visible on demo). Some themes has language selector, some themes use user selected locale only.
Theoretically you can check one of free themes that has selector and copy it into your theme (it's generated by osclass, not theme).
You should see code like this:
<?php ItemForm::multilanguage_title_description(); ?>
On the other side more preferred option is user selected locale only. In that case we have split title & desc:
<?php ItemForm::title_input('title',osc_current_user_locale(), osc_esc_html( sigma_item_title() )); ?>
<?php ItemForm::description_textarea('description',osc_current_user_locale(), osc_esc_html( sigma_item_description() )); ?>