If the app interface has changed to Russian, it is because of the preferred language order in your device settings. It is likely that you have Russian in second place after your native language, e.g. German (or any other language(s) apart from English). As there is no German interface in the app, for example, but there is Russian and it is listed as the next preferred language, the app's interface switches to Russian.
In order to make the English interface appear, you should set English as your second preferred language (or move it just higher than Russian). To do this, go to Settings > General > Language & Region > Preferred Language Order. Then tap on 'Edit' to reorder the languages.