There are four main reasons for it.
Spiritually, it brings us close to a certain earthly aspect of the Mother’s personality. It is the same logic as of learning Gurumukhi (Punjabi) emphasised by the Sikh Gurus, Braj bhasa by devotees of Krishna, Sanskrit for reading the Upanishads etc.
There is an occult dimension of Speech which is meant to carry vibrations through the sounds. So reading or even better, hearing The Mother originally in French certainly carries those vibrations deeper than reading Her works in English just as reading Sri Aurobindo in English helps better than in translated languages where much is lost.
Learning new and different languages is known to develop the Intelligence and increases the much needed plasticity and wideness.
For students She saw that learning French will hold them in good stead for subsequent opportunities.
Of course the closer ties of India and France are very much needed part of the New World Order.
Affectionately,
Alok Da
