I have the French version for 4 months now and it works as perfectly as the English version I had before.
That being said, I was told by HP that it was not possible to replace the English version with a series of French disks. I guess it is the same the other way around, like you would like to. I was told to contact the seller, which I did. Well the seller told me I had to return the whole machine to replace it with a French model (it is the exact same machine, except for the system software), I didn't do that. So I went to the store, they happened to have a French machine and wanted themselves to hold an English version. The exchange of disks was quite easy.
So I guess you'll have to try with stores or return the French machine for a new English one.
I'll wait t'ill the 4th of August for PP1.
Edit: Besides all this I would advise that the language of your MSS matches the one of the computers on your network. Read the two last paragraphs of a former message
here