Example of a custom functions plugin that currently has a string translation function within it.

ee-custom-functions-plugin.php

<?php
/*
Plugin Name: Event Espresso Custom Functions Plugin
Description: Add custom functions for Event Espresso to this plugin.
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */

function tw_ee_mycustom_filter_gettext( $translated, $original, $domain ) {
 
    // This is an array of original strings
    // and what they should be replaced with
    $strings = array(
        'The event you have selected requires logging in before you can register. You can %sregister for an account here%s if you don\'t have a login.' => 'The event you have selected requires logging in before you can register. <strong>New Users</strong> you can %sregister for an account here%s if you don\'t have a login. <h3>Returning Users</h3>',
        // Add some more strings here
    );
 
    // 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;
}
 
add_filter( 'gettext', 'tw_ee_mycustom_filter_gettext', 10, 3 );