There isn't, by design. You write _some_ black-box implementation locally which outputs a textual solution you drop into the browser (at least, this is how it was in 2015 when AoC started).

I can't recommend this enough. I had to miss out on last year's challenges, but planning on participating this year; it's a great way to keep your brain nimble if you feel like you're in a rut.

Nope! All of the answers boil down to some simple text input, so you can run whatever code you want locally to get the answer.

> is there a specific language used for these?

There isn't. Some previous tasks could be solved by pen-and-paper, or by using your browser's search on the input, etc.

Also, there were some users who solved each day in a different programming language.