French Prime Minister Manuel Valls told Le Monde in an interview that this ban would be "indefinite" and is part of the country's expanding counterterrorism efforts. France is also working on improving relations with Islam.
From The Guardian:
In an interview with French newspaper Le Monde, Valls said he was “open to the idea that – for a period yet to be determined – there should be no financing from abroad for the construction of mosques”.
The prime minister also called for imams to be “trained in France, not elsewhere”.
He said the interior minister, Bernard Cazeneuve, whose portfolio also includes religious affairs, was working on building a new model for France’s relations with Islam.
Interesting, and not entirely unprecedented. Norway has prohibited "countries that do not permit religious freedom" (namely, Saudi Arabia) from financing mosques in the country. Austria passed a similar law in 2015, and Italy is considering a ban as well.
Of course, France could also start by ending the absurd practice of allowing convicted terrorists unsupervised release periods.