Query posts by Post title

That query has nothing to do with ACF. It’s a regular post query using WordPress’ standard query class. As such you can refer to the documentation for the options for ordering your query.

To sort by post title you just need to set orderby to title:

<?php
$args = array(
    'posts_per_page' => 999,
    'post_type'      => 'lezing',
    'orderby'        => 'title',
    'order'          => 'ASC',
    'meta_query'     => array(
        array(
            'key'   => 'jaar',
            'value' => '2019',
        ),
    ),
);

$the_query = new WP_Query( $args );

Also note that the quote marks in your original question are incorrect. You need to use non-fancy quotes like '. This can be an issue if you’re copying code from incorrectly formatted blog/forum posts.