Issue in administration of menus

Hello, please it looks like we have found a small bug. Example – I have choosen to show only EN langage content -> go to appearance -> menu -> now if I click the save menu button it always switches automaticaly after save(refresh) to Russian language content (the flag on the top) and also the menu structure and other fields (whole adminstration of the menu) is written in RU (Структура меню, Открывать в новой вкладке, или создайте новое меню. Не забудьте сохранить изменения! and so on…) PLEASE we cant find any bug on our side, is there a known issue with this – or how to solve this mistake please…? Thank you a lot for answer, Petr