File a bug report if you're having problems with what should work normally. Just make a CAPX showing it works normally (text object) and that when loading from AJAX, strings display properly except those characters.
Well, before reporting a bug make sure if there isn't an issue with the txt file, it's maybe its encoding that is wrong. Maybe try to save the .txt to UTF or something like unicode to see if the issue does not come from there.
Construct 2 fully supports unicode. You're probably saving the .txt file with the wrong encoding. Make sure you save it with UTF-8. If you save it as ASCII, non-english characters will appear as the wrong characters, which explains the issue.