solution

Here's an example implementation of this solution:

public function test_get_pages_with_taxonomy_on_page() {
    wp_set_current_user( 1 );

    register_taxonomy( 'language', 'page' );

    $term_en = self::factory()->term->create( array( 'taxonomy' => 'language', 'name' => 'english' ) );
    $term_fr = self::factory()->term->create( array( 'taxonomy' => 'language', 'name' => 'french' ) );

    $page_en = self::factory()->post->create( array( 'post_type' => 'page', 'tax_input' => array( 'language' =>  array( $term_en ) ) ) );
    $page_fr = self::factory()->post->create( array( 'post_type' => 'page', 'tax_input' => array( 'language' =>  array( $term_fr ) ) ) );

    add_filter(
        'get_pages_query_args',
        function ( $query_args, $r ) {
            $query_args['tax_query'][] = array(
                'taxonomy' => 'language',
                'field'    => 'slug',
                'terms'    => $r['language'],
                'operator' => 'IN',
            );

            return $query_args;
        },
        10,
        2
    );

    add_action(
        'parse_query',
        function ( $query ) {
            if ( ! isset( $query->query_vars['language'] ) ) {
                $query->set( 'language', 'english' );
            }
        }
    );

    $pages = get_pages( array( 'language' => 'french' ) );

    $this->assertCount( 1, $pages );
    $this->assertSame( $page_fr, $pages[0]->ID );
}