On the web site allow to register user and let the user log-in.

On the mobile application, let the user just use it or to create
a username (without password) just by typing in the name.
Then allow to swicth between users.

Each user can have one or more learning objectives.

Select two languages (source and target).

Allow to add word pairs in the two languages.

A lesson can hold a set of words. (lessons can be numbered)
Then we show one of the words in the source language and the user needs
to type the word in the target language.

At first the whole thing should start as a pure client side application.