The UI for Luma is in English but there are a lot of places in Luma that you can customize to be in your own language.
Event Pages
Event title, description
Event emails — add custom text to registration confirmation, reminders and more
Profile
You can customize nearly everything on this page
Newsletter Messages
You define all of the content in your newsletters except for the footer
We don't have plans to support translating Luma into other languages. But we are working hard to add more flexible customization.