Yes, any international airport - especially a large and very busy airport - should use a single language for all ATC communications for solid safety reasons.
CDG has already been the site of a fatality when the wing of an aircraft - cleared to take off in French - killed an English speaking pilot of another aircraft - a situation that could have been realised and prevented had a single language been in operation.
It is madness to allow two languages to control such a high density and busy airport.
As to your point 1, yes, in theory, but as you know; GPS is not infallible either and can be jammed etc. Where is the GPS altitude read-out? is it on the PFD or on a page somewhere in the MCDU? We already have a well practised method to check and cross-check the QNH, and three independent pressure sensing altitude read-outs in the cockpit to cross check between, as well as the ATIS.
The mistake in this case was a mis-translation by ATC of the QNH from one language to another, and the aircraft in question either not following the standard QNH cross-check or not querying it.