Filter content in shortcode

The year attribute you added to shortcode [speaker_overview_2017 year=2017] will be available through $atts in this way $atts['year']. I assume, you want to do a meta query and the key is speakers_jaar and the value will be $atts['year']. Please check the following code. When a year is set in shortcode, query will be limited to that year otherwise not limited.

function speakers_overview_shortcode( $atts ) {
    $atts = shortcode_atts( array(
        'year' => ''
    ), $atts );

    $args = array(
        'post_type'      => 'qp_speakers',
        'post_status'    => 'publish',
        'orderby'        => 'menu_order',
        'order'          => 'ASC',
        'posts_per_page' => -1,
    );

    if ( ! empty( $atts['year'] ) ) {
        $args['meta_query'] = array(
            array(
                'key'   => 'speakers_jaar',
                'value' => absint( $atts['year'] ),
            )
        );
    }

    $locaties = new WP_Query( $args );

    ob_start();

    if ( $locaties->have_posts() ) :
        while ( $locaties->have_posts() ) {
            $locaties->the_post();
            q2_speakers_template();
        }
    }
    wp_reset_postdata();

    return ob_get_clean();
}
add_shortcode( 'speaker_overview_2017', 'speakers_overview_shortcode' );