Josh Ge is the creator of Cogmind and moderator of the roguelike-dev subreddit. His talk is advice, not a tutorial.
What language? Well, any language works but he recommends Python to get started. A Python library called libtcod provides a starting point with basic functionality.
The key thing is, like a tech startup, to find the minimal path to the core dynamic. Build something that's fun first, and add other modules later. He gave various 7DRL examples of fun, unique mechanics that did not take a lot of time to demonstrate....but a beginner probably shouldn't jump in to this seven-day competition as their first experience.
Design: finding a hook. Core mechanic? Sound/VFX? How about a unique theme! We already have a ton of dungeon crawlers. makaiRL, Skies of Bloody April, Lone Spelunker. Pirates? Superheroes?
Armoured Commander is a great example of what is possible from zero experience to a game featured in RPG a year later.
XRL: based on existing IP, but don't pick Nintendo properties, and expect a C&D letter at some point. Built-in fan base.
(bSteem wouldn't let me post so I copied and pasted into the web app instead.)