If you go to make a payment, and the language suddenly changes on you, don't panic! This is a known issue that we are actively working to solve.
Tl;dr: The Legends of Runeterra client is reacting to the set language of your region rather than your preferred language. This means that if I'm in Japan, but my preferred language is Turkish, the payment window may appear in Japanese!
We apologize for any confusion this may have caused, but we're working to get you to the right language ASAP.