I am currently writing (translating) ux content for a web app that targets children who want to practice languages. Our UIUX designer decided to come up with 3 types of language systems, one is interface language, another is the user's mother language and the last one is the language they want to learn. Her purpose is to let users freely choose the interface language they are familiar with. In the app, when users are learning languages, the system will show their mother language ( depending on the sections).
For example, based on the uiux's case study: the user is a Japanese learning English. She is already familiar with English so she will set up the interface language as "English". When she is learning "English" in our app, some of the sections will be displayed in Japanese ( such as the learning content, translation, explanation, etc) but the UX writing such as guiding user "how to learn" and interactive buttons are still in English. The way she distributes which section belongs to mother language or interface language I find it very confusing because the system is showing multiple languages at the same time. Should I suggest she merge both interface and mother languages as one only?