How can I override strings? I'm trying to change a label of Drupal 8's contact form ('Your email address'). The label is defined /core/modules/contact/src/MessageForm.php, but obviously I don't want to change this in core.

$form['mail'] = array(
'#type' =&gt; 'email',
'#title' =&gt; $this-&gt;t('Your email address'),
'#required' =&gt; TRUE,
);


In Drupal 7 I always used string overrides but the module isn't available for Drupal 8 yet. How can string be overridden in Drupal 8?
The easiest method would be to implement a hook_form_alter inside a custom module that overrides the title 

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.2.x

Some information how to create custom module.

https://www.drupal.org/docs/8/creating-custom-modules