Ok, I managed to select whole accented words double clicking on them...
At first I tried to simply change the encoding using :
this.scintillaControl.Encoding = Encoding.UTF8
But it doesn't seem to work properly... Instead I changed the Encoding directly during the control initialization using :
_ns.SetCodePage(65001); Instead of _encoding = Encoding.GetEncoding(NativeInterface.GetCodePage());
Now it's really UTF-8, but the caret position still show ANSI position when I test with :
He'd after the apostrophe it indicates 5 instead of 3.
Don't know if there's something else to modify in order to make it work ?
Anyway the initial problem is solved ! :-)