Yes, we have a theming system that lets you change the UI… these guides may help:
Beginner's guide to using Discourse Themes
Developer’s guide to Discourse Themes
For languages, you can change the default UI language using the admin setting called default locale and on an individual level users can change their language in their preferences.
Our translations are crowdsourced from the community, and the list of available languages (and their completion) is available here: Crowdin Enterprise