No! Anyone can participate and build projects with whatever tech stack they feel comfortable with. Participation is open to all stacks and all languages. If there are more projects on Python or ML, I’ll get mentors to review them as well. And winning doesn’t depend on stack either.
Code in any language/stack of your choice.
But make sure that all the projects are functional and useful. Even if you are only familiar with a particular stack (front-end or back-end), you can build half stack projects.
What are half stack projects? Let me give you an example. An API you made which gives random quotes. Good. But did you document it? Did you make a documentation site for it? Did you provide enough examples for end users to use it? Did you ask people in this group to build on that API using your docs? This makes a half stack project complete.
Let me give you another example. Suppose you made a dashboard theme. Good. Great! But did you make it configurable? Do you have a plain HTML version and a React version? Again, did you document it well?
Make a half stack project complete by documenting and asking other developers to build on it.