What's Next: C++, C#, and Java, and Why They're Not Here Yet
LTWC's full lesson chain runs seven languages deep. Coders Rock ships the front half first, and here's the plan for the rest.
Learn to Write Code's lesson chain runs Concept, Pseudocode, Python, JavaScript, C#, Java, C++. Coders Rock's v1 covers the front half -- pseudocode, Python, and JavaScript, all running in your browser with no install. The back three are planned, not abandoned, and each has a specific browser-based runtime picked out rather than a vague someday.
C++ is the closest: JSCPP is a pure-JavaScript C++ interpreter built for teaching, free and light, and it covers the cout/if/loop level these lessons actually use. C# would run on Blazor WebAssembly -- real .NET in the browser, free, Microsoft-maintained -- though it's the heaviest of the three to load, so it may end up scoped to just the pages that need it. Java would run on CheerpJ, a mature, actively developed JVM-in-the-browser project; its free community licence covers a one-person company like this one with no cost, on the condition that the runtime loads from CheerpJ's own hosted CDN rather than being vendored the way the Python interpreter is.
That CheerpJ condition is worth being upfront about: it would make Java challenges the one place on this site where a third party sits in the request path to load the runtime, even though your code itself still never leaves your browser once it's running. Until any of the three ships, the honest fallback -- already used elsewhere on this site -- is showing the code as a read-only reference rather than shipping a Run button that doesn't actually work.
Everything above runs in this page, on your own machine. There is more of it on the practice pages.