<?php
header('Content-Type: text/plain; charset="utf-8"');

$input = '[{"id":"1","site":"7","elemId":"navbar_title","en":"Home","de":"Zuhause","fr":"accueil"},{"id":"2","site":"7","elemId":"home_untilNextTime","en":"Until Next Time","de":"Bis zum n\u00e4chsten Mal","fr":"Jusqu\'\u00e0 la prochaine fois"}]';
$raw = json_decode($input, true);

$output = array_map(function ($entry) {
    return [
        'en' => $entry['en'],
        'de' => $entry['de']
    ];
}, array_column($raw, null, 'elemId'));

echo json_encode([$output], JSON_PRETTY_PRINT);