The majority of courses on memrise are in English. These courses seem to be only available when the display language is set to English.
The android app defaults to the language selected for the OS and there is no option to change the app language. Many people learning a language set their phone to the new language. Unfortunately, the majority of the courses on memrise vanish when the OS language is not English. Even searching for the exact course names (in English) shows nothing.
A simple fix would be to add an option to change languages in the memrise app. Or just make all courses searchable no matter the display language.