Polylang: Translate custom strings

functions.php

You must first register the strings for translation.
For example you echo "Hello world" in some template file like this:

<?php pll_e('Hello world'); ?>

To show string in the "Strings translation" add in your functions.php:

<?php
  add_action('init', function() {
    pll_register_string('mytheme-hello', 'Hello world');
  });
?>

Add all custom strings you want to translate to this function.