1. Translating the Theme to a different Language
You can quickly translate much of the theme by simply changing the 'Site Language' via Settings > General > Site Language
2. Changing Text Strings
Jefferson comes with a translatable .po file within the /languages directory. There are lots of tutorials online about using Poedit to translate such a file.
One of the easiest approaches is to use the Loco Translate plugin. We've written a guide about using it on our blog, available here.
You can use it to 'translate' strings in the same language also - so English strings can be changed also.