Java, Ruby, PHP - all safe bets for server-side language. They're all easy to find developers for, stable, and easily supported by all major IaaSes (AWS, Google Cloud Platform, Azure) and PaaSes (Cloud Foundry, Heroku, Kubernetes, Docker).

Javascript - either no Javascript, or very simple vanilla Javascript. No transpiling, no Typescript, no JSX, no ES2015.

CSS - Regular ol' CSS. Set a limit on number of lines of CSS you're willing to introduce per controller action and try and keep your project honest to that number. Avoid frameworks if possible, pick a simple framework for grid layout if you really need it.