Is spell numbers are the same in english than in French, for example...
Then he wouldn't have any reason to disconnect...
And even if he had a different 'version' (i mean, if french version is late of 1 patch next to english one, ie.: complete healing is 10 seconds on french one, when it is 9 on english one), he would have his client GUI *bugged*, but all woudl work fine anyway...
And for the language thing, don't change the language *.ini or whatever it is on client, just connect with renamed files
Good luck !