Which programming language should you learn if you want a job at Google, Amazon, Facebook or any big software company ?

If you're good at designing algorithms, you do not need to know multiple languages. One would do. At companies like Google and Amazon, solving problems is the main focus. For this, being proficient at making algorithms is key. I've interviewed several candidates over time who have unique programming skills but zero problem solving skills. You can know 4 languages from top to bottom, but if you're a dud at designing algorithms, chances are you won't be hired. So be proficient in any programming language of your choice (preferably Python), and be a prolific algorithm mapper.