Translation of site title, description and metatag

Here I’m tiring to translate site.yaml footer: description: of the deliver template !
So far i try to follow your advice it seam i miss something as it dos not work.

I change in the file :

In user/config/site.yaml

footer:
  description: 'Bla bla bla'

for

footer:
  description: 'SITE.FOOTER.DESCRIPTION'

In user/themes/deliver/templates/modular/footer.html.twig

            </div>
            {% if site.footer.description %} 
                <p>{{ site.footer.description }}</p>
            {% endif %}
        </div>

for

            </div>
            {% if site.footer.description|t %} 
                <p>{{ site.footer.description|t }}</p>
            {% endif %} 
        </div>

added user/languages/en.yaml

SITE:
  FOOTER:
    DESCRIPTION: 'My description'

added user/languages/fr.yaml

SITE:
  FOOTER:
    DESCRIPTION: 'Ma description'

What i get in the website is a text saying : SITE.FOOTER.DESCRIPTION

Which is obviously not what i want

What do I missed ? What do I do wrong ? Thank you very much for our help !