Your question is a bit open-ended.
Scaling your database is generally the hardest part. Any jerk can put 100 app servers behind a load balancer. Scaling data is significantly harder, especially when it comes to writes. There are options to get around this, but they come with compromises.
So, really, it depends on what you're doing. In general, use things you're comfortable with. Is Python slower than Rust? Sure. But do you need 2ms response times from your app server or is 10ms fine, especially when latency between the user and your first load balancer is likely 3-5x that number? Build it, test it, if it's too slow, cache it. If you can't cache it, break out the offending piece and optimize it or put it into a faster language.