Free Rice: Fight World Hunger and Learn English for Free! -The UN World Food Programme
FreeRice is an amazing learning app of many subjects and languages that you can learn. The more correct answers you get, you can feed children in need faster, and make groups with your friends to get high scores. The English vocabulary sections have so many levels of difficulty that you can challenge.
The categories I recommend for English language learning are:
The Global Goals
Healthy Eating for Goals 2 & 3
The Nobel Peace Prize