When exporting your project, you get a "www" folder, and in it you will find the "index.html" file
When you open it with a text editor such as SublimText, you will see the line "<meta charset="UTF-8" />" which is equivalent to what I wrote before (but you can still try to replace it, it might maybe help on some browser)
It seems Construct2 already encoded the file in UTF-8, so it's more probable there is a problem with the encoding of the text file you read with ajax, so you should try saving it with another encoding
In sublimText2, you use "File > Save with encoding" and you select your encoding (here UTF-8) It might mess up every accent in your document though, but it's the only way. (this software exists in a trial version you could use if needed)
I'm not an expert but I'm pretty sure the problem comes either from the fact that the web page's encoding is wrong and can't display the characters correctly, either from the fact that you try to load text from a file with ANOTHER encoding and the characters are not converted properly ^^' To avoid this kind of problem, you should always use the same character encoding in every file