The following code (with appstream-qt):

AppStream::Pool pool;
pool.load();
AppStream::Component c = pool.componentsById("org.kde.krita.desktop").first();
qDebug() << c.description();

Gets the description (of Krita, in this case) in whatever the default language is; setting the environment variable LANGUAGE to fr, for instance, gets the French description.

There doesn't seem to be a way to get the description for any other language, though: setActiveLocale() on the component doesn't change the text that is returned. Calling it on the pool before load does load the other language -- but only that one.

So how does one get the description of a component, in all the languages for which a description is available for that component?

(something similar might be said about appstreamcli dump .. it only dumps the name and description in the current language, not all of them)