ff-submit-btn-state-content.php

// required css
<style>
span.ff_submitting {
    display: none;
}

.ff-working  span.ff_submitting {
    display: block;
}

.ff-working  span.ff_on_normal {
    display: none;
}
</style>

<?php

add_filter('fluentform_rendering_field_data_button', function ($data, $form) {
    $data['settings']['button_ui']['text'] = '<span class="ff_on_normal">' . $data['settings']['button_ui']['text'] . '</span><span class="ff_submitting">Submitting</span>';
    return $data;
}, 10, 2);