Sorry for being pedantic, but what does the "L" stand for in HTML, YAML, SQL?
They may not be "programming languages" or, in the case of SQL, a "general purpose programming language", but they are indeed languages.
Recently I've been experimenting with using multiple languages in some projects where certain components have a far better ecosystem in one language but the majority of the project is easier to write in a different one.
For example, I often find Python has very mature and comprehensive packages for a specific need I have, but it is a poor language for the larger project (I also just hate writing Python). So I'll often put the component behind a http server and communicate that way. Or in other cases I've used Rust for working with WASAPI and win32 which has some good crates for it, but the ecosystem is a lot less mature elsewhere.
I used to prefer reinventing the wheel in the primary project language, but I wasted so much time doing that. The tradeoff is the project structure gets a lot more complicated, but it's also a lot faster to iterate.
Plus your usual html/css/js on the frontend and something else on the backend, plus SQL.
Go for the backend, something javascripty for the front end. You're already at two. Depending if you count HTML, CSS or SQL as "languages", you're up to a half dozen pretty quick.
3 or 4 can very easily accumulate. For example: HTML, CSS as must know, plus some JS/TS (actually that's 2 langs!) for sprinkles of interactivity, backend in any proper backend language. Oh wait, there is a fifth language, SQL, because we need to access the database. Ah and those few shell scripts we need? Someone's gotta write those too. They may not always be full programming languages, but languages they are, and one needs to know them.
