Remember that google and facebook aren't all there it. There are lots of software companies, and I don't know in your country, but in mine, they tend to look for people that code in: Java, PHP, Front end, and C++. In that order. You should analyze job opportunities in your country to have an idea what language gives you the most opportunites
As noname said and if you really intend to work in these companies you must have really strong concepts and problem solving skills, language you use doesn't matter much.
As for building apps, of course kotlin is better, since google have made it official language for Android and it saves a lot of headache.