Open the FAF client.
At the top-left, there is the "hamburger" button (three horizontal lines, called a "hamburger" because the three lines are like 2 buns and a patty)
click on hamburger menu
go down to "settings." Which would probably be called something else. But you can tell which one it is because it's the second-to-last option. It's the one just above "exit"
then you have to click on the option for "language" and you will have a list of 15 languages. Pick the one you want.