This is probably not the popular opinion in this crowd BUT PHP is still the most used back-end language on the web. It has also come a long way since it's early days. Modern PHP is actually decent to work in and learning it will keep you gainfully employed in most areas outside the Bay Area.

For back-end, Python & PHP are still my go to choices. Both are stable, have tons of libraries, have good frameworks and allow you to get stuff done quickly.