That isn't true, French regulation allows for the import of homebuilt ULMs. There is of course a big difference between theory and practice. As with most countries the process is best avoided. The detail and the required provenance is demanding and therefore very risky. There is always the likelihood that your aeroplane will be rejected with a demand that it is impossible to satisfy.
The RV series are perhaps the most sought after of home built aeroplanes. Difficult to find but quick to sell. The best advice is to buy a ULM/Microlite registered within the country where it is intended to operate at least most of the year. Buy an aeroplane that thorough research assures a quick sale should it become necessary.