Example. A function used to translate choices of the Checkbox field in the Ultimate Member form. Uses the Polylang language switcher.

um_checkbox_choices_translate.php

<?php

// Translate choices of the Checkbox field.
function my_checkbox_item_title( $title, $key, $v ) {
	$lang = pll_current_language();

	$texts = array(
		'um_pet' => array(
			'en' => array(
				'Cat'  => 'Cat',
				'Dog'  => 'Dog',
				'Bird' => 'Bird',
				'Rat'  => 'Rat',
			),
			'es' => array(
				'Cat'  => 'Gato',
				'Dog'  => 'Perro',
				'Bird' => 'Pájaro',
				'Rat'  => 'Rata',
			),
		),
	);

	if ( array_key_exists( $key, $texts ) ) {
		if ( array_key_exists( $lang, $texts[ $key ] ) ) {
			if ( array_key_exists( $title, $texts[ $key ][ $lang ] ) ) {
				$title = $texts[ $key ][ $lang ][ $title ];
			}
		}
	}
	return $title;
}
add_filter( 'um_field_checkbox_item_title', 'my_checkbox_item_title', 10, 3 );

function my_checkbox_item_title_view( $value, $data, $type ) {
	if ( 'checkbox' === $type && is_array( $value ) ) {
		$key = $data['metakey'];
		foreach ( $value as &$title ) {
			$title = my_checkbox_item_title( $title, $key, $title );
		}
	}
	return $value;
}
add_filter( 'um_profile_field_filter_hook__', 'my_checkbox_item_title_view', 10, 3 );

Choices in the Ultimate Member fields are not translatable. You can use this workaround to translate choices of the Checkbox field.
In this code I translated choices of the Pet um_pet field. Change the $texts array for the field and choices you need.

The field settings example

You can use this solution in addition to the the Ultimate Member - Polylang plugin.

Related:

• How to translate plugin
• How to translate choices of the Dropdown field