That will probably improve the usability quite a bit for the many users who have to switch between two or more languages several times a day.
My worry is that it is not perfectly clear what that link does, though. Since it looks a lot like a normal hyperlink, it could as well navigate to another page that contains the demo text. That could be improved by
- making it say "Insert demo text", or
- making it a button element with the same style as the "Check Text" button.
I prefer the latter, because it is more in line with the traditional design principles: buttons trigger an action, often on the same page, links are for navigation.