These are not meant to be the only projects you can do, or complete ideas that can't be modified. They are just here to give some idea what the project might look like, how big it should be, etc.
There are many tools to convert various text formats between one another. The likely input is a format that an author might want to write: WikiText, Markdown, reStructuredText, LaTeX. Output would likely be a format compatible with appropriate viewers: HTML, PDF, EPUB.
Any kind of system that can convert a reasonable subset of these is going to have to use libraries from multiple languages, one way or another.
At some point, you might also have to face the fact that some of these languages aren't the fastest at actually processing text. It might be worth using something else (C++, Rust, Go) to do some of the work because it's “faster”.
Calculation on the Web
In this scenario, I'm going to imagine that the user in the web browser is going to be shown some information that is the result of a large calculation. The languages that are common choices to implement REST APIs aren't generally the fastest at pure computation. We can incorporate something “faster” (like C++, Rust, Go) to do the hard work. [For this project, it doesn't really matter if the calculation you're doing is actually that complex; it's perfectly reasonable to do some smaller calculation that makes sense, use that as an excuse to switch to a different language, and describe in the
README that it could be more complex in some future version.]