The basic rule is: system language will be changed by switching courses.

For English speakers: 

You can select the language you want to learn by tapping on the national flags>>tap on "Course for English speakers">>Select the course. The system language will switch to English.

For non-English speakers:

Simply tap on the English course with American flag>> tap on "Courses in more languages" >> Row down and find the language you want>> tap on the course >> System language changed!