anamethatisnt

In GNOME it's as simple as shown in the image: Settings -> Region and Language:

And here's an explanation of the value as well as an explanation of why it is set to false by default:
https://firefox-source-docs.mozilla.org/intl/locale.html#regional-preferences

Tl;dr:
Example: It might lead to a UI case like “Today is 24 października” in an English Firefox with Polish date formats.

Their metadata provider system by default only has The Movie Database (TMDb) and Open Media Database (OMDb) but they got plugins for more:
https://jellyfin.org/docs/general/server/plugins/#official-plugins