"I am reading a menu."
Translation:Je lis un menu.
In English, to mean that an action is in progress at the time you speak, you use the continuous verbal form, ie verb BE + action verb in the gerund form (-ing). o he is eating means he currently eats
In French, this verbal form does not exist (directly translated “il est mangeant” is incorrect).
Therefore, you can translate either “il mange” or “il est en train de manger”, where the construction verb être + en train de + infinitive correctly expresses the English continuous form.
Je suis = I am
Je = I
You would use "je" + a verb. E.g. je mange, je dors, je lis, je suis (I eat, I sleep, I read, I am) etc.
Or maybe I'm misunderstanding your question.