I have the possibility of getting a good deal on a new iMAC.

But the problem is I'm buying it in France.

So it will be in French.

My question is is this easy to change to English if the OS has been already installed in French?

In system preferences under the "international" tab you can set the primary language for the OS, finder and the rest of the user interface.

So I should be OK if I pick up this French iMAC?

Meaning to change the language settings to English should be possible!

Yes Simon.

The language the computer displays has NOTHING to do with the hardware.

It's all about the software.

And on Macs, the OS software is compatible with most modern languages.