There are several things to consider.
Would putting 10-40 in instead of 0-30, cause a engine failure, very unlikely, in fact there may well be conditions where it is recommended such as operations in a hot climate.
Could putting any type of wrong oil in cause an engine failure, possibly. I've seen the results of SAE100 axle oil being put in a Tranny engine, it broke the oil pump drive. Also putting a oil in that wasn't suitable for a turbo engine has been the cause of many turbo failures. There is also a remote chance a totally unsuitable oil was put in such as hydraulic oil, unlikely but sometimes these things happen.
Lastly why would the French mechanics say the wrong oil was used. I can think of two reasons, firstly it was a tottaly unsuitable oil and obviuosly so, or was something 'lost in tranlation', did he actually say the wrong amount of oil had been used.