Translate strings with context. Important: use this filter function only for i18n-ready strings that are wrapped in esc_html_x() or similar.

gettext_with_context_example.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter( 
    'gettext_with_context', 
    'mycustom_filter_gettext_with_context', 
    10, 
    4 
);
function mycustom_filter_gettext_with_context( 
    $translated, 
    $original, 
    $context,
    $domain 
) {
    // This is an array of original strings
    // and what they should be replaced with
    $strings = array(
        'Attendee %d' => 'Participant %d',
        // more strings with context here if needed
    );
    // See if the current string is in the $strings array
    // If so, replace its translation
    if ( isset( $strings[$original] ) ) {
        // This accomplishes the same thing as __()
        // but without running it through the filter again
        $translations = get_translations_for_domain( $domain );
        $translated = $translations->translate( $strings[$original] );
    }
    return $translated;
}