TLDR: C++, WASM, and some form of GRPC with C++ on the server side as well. Because you need a language that's fast, can contain high complexity and large programs without collapsing (which is a short list of languages) and can work fast for the bits that need speed.
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Something like this that's proven itself: https://www.figma.com/blog/webassembly-cut-figmas-load-time-...
TLDR: C++, WASM, and some form of GRPC with C++ on the server side as well. Because you need a language that's fast, can contain high complexity and large programs without collapsing (which is a short list of languages) and can work fast for the bits that need speed.