Each month a PLT Game theme is announced. Developers start working on a language that fits the theme and submit their projects. At the end of the month (midnight UTC) submissions are closed and entries can be rated during the next month.
PLT Games was created to encourage small experiments in programming language:
Entries must relate to the month's theme. A theme is something fairly open, such as:
Anyone can sign up to rate entries during the month after the competition. Entries are rated along the following attributes:
Top rated entries are announced at the end of the rating month.
No. A programming language that just exists is not a problem. It can only become a problem when that language is used seriously. These experiments aren't to be taken seriously unless they deserve to be.
It can run wherever you want. Entries will be judged so it's best to make it easy for the audience to play around with to get good results.
Developers aren't expected to spend a full-time month working on their language (good on them, if they can). The month time-frame is to give developers a chance at a weekend or a handful of nights working on their language.
A month is quite short to create a non-trivial programming language but that's not the goal. The goals are to make something small and interesting. This should be possible by creating a very simple parser in a few hours then creating an AST interpreter. That's very worthy of a submission.
Yes, you can use any existing code that was developed before the competition. You could even take Ruby's source and modify it, if you felt that was the best way to make something that suited the theme.