"Learn single language build everything". Lets suppose you want to become mobile app developer and back-end as well. So should have following..
1-java (Front-end)
2-xml (Front-end)
3-Swift/Objective-C (Front-end)
4-php/node (Back-end).
5-mysql,json/NoSql (Back-end).
But If you have knowledge of JavaScript this is enough for all kind of development. React-native/Ionic/Node/NoSql/JSON all is the form of JavaScript.