Extension:Translate question: how do I translate non-MediaWiki content?

I've installed Extension:Translate on my own wiki, and I want to translate some non-MediaWiki content (software interface text, in particular). How do I configure that, and what file formats are supported?

Also no, I don't feel like the project can/should be hosted on translatewiki.net.

See

• https://www.mediawiki.org/wiki/Help:Extension:Translate/Group_configuration
• https://www.mediawiki.org/wiki/Help:Extension:Translate/Group_configuration_example