Promotions not translated on cart and checkout page?
I found work-around for this issue. Inside the template that prints that section (commerce-order-total-summary.html.twig) there is a loop that prints adjustments:
{% for adjustment in totals.adjustments %}
    &lt;div class=&quot;order-total-line order-total-line__adjustment order-total-line__adjustment--{{ adjustment.type|clean_class }}&quot;&gt;
        &lt;span class=&quot;order-total-line-label&quot;&gt;{{ adjustment.label|t }} &lt;/span&gt;&lt;span class=&quot;order-total-line-value&quot;&gt;{{ adjustment.amount|commerce_price_format }}&lt;/span&gt;
    &lt;/div&gt;
{% endfor %}

and where adjustment label is printed I added t twig filter, so now it can be translated on standard user interface translations page:
/admin/config/regional/translate

I know that it's not optimal solution, but it works, so I hope this can help someone until bug is fixed.