Drupal 8: How to put a comma in addressfield display?
It appears you would need to write a custom address-plain.html.twig file and add it to the templates folder either in a module or in your theme.

Code: 

&lt;p class="address" translate="no"&gt;
  {% if given_name or family_name %}
    {{ given_name }} {{ family_name }}, &lt;br&gt;
  {% endif %}
  {% if organization %}
    {{ organization }}, &lt;br&gt;
  {% endif %}
  {% if address_line1 %}
    {{ address_line1 }}, &lt;br&gt;
  {% endif %}
  {% if address_line2 %}
    {{ address_line2 }}, &lt;br&gt;
  {% endif %}
  {% if dependent_locality.code %}
    {{ dependent_locality.code }}, &lt;br&gt;
  {% endif %}
  {% if locality.code or postal_code or administrative_area.code %}
    {{ locality.code }} {{ postal_code }} {{ administrative_area.code }}, &lt;br&gt;
  {% endif %}
  {{ country.name }}.
&lt;/p&gt;