2Planks:
Or maybe the J2 ac was on the right frequency and French ATC had forgot to tell them to change and then wonder why they have lost them.
HT:
Doubt it. The next sector expecting the aircraft on their frequency would check with the previous to remind them. If no contact they would then try to contact the aircraft on 121.5
2Planks has probably the closest guess out of all the responses here, it does happen, and enough to be considered regularly.
737 also said that Brest tried calling said aircraft for 20 minutes, further indication they were at least not on the expected frequency (if they were on a frequency), and RVF750 said that there were some accents hard to understand that their FO didn't quite understand. It is possible for the aircraft to have dialled in a wrong frequency, checked in as normal, and for the ATC unit to acknowledge without realising that the aircraft should not be on their frequency. Again, it is something that happens and has caused this situation in the past.