How can I get field's value of entry in other site locale in my element-api endpoint?
Okay I found a way to do it if it can help someone: 

$description_fr = ($entry-&gt;summary) ? $entry-&gt;summary : ($entry-&gt;body) ? $entry-&gt;body : '';
$entry_en = Entry::find()-&gt;id($entry-&gt;id)-&gt;siteId(2)-&gt;one();
$description_en = ($entry_en['summary']) ? $entry_en['summary'] : ($entry_en['body']) ? $entry_en['body'] : '';
$entry_de = Entry::find()-&gt;id($entry-&gt;id)-&gt;siteId(3)-&gt;one();
$description_de = ($entry_de['summary']) ? $entry_de['summary'] : ($entry_de['body']) ? $entry_de['body'] : '';