TL;DR: Search twice or more before you make a new plugin. The one you need may just be hiding in the plugin repository.
Sweetz, am I happy when I’m tweeting this:
— Caspar⚡️Hübinger (@glueckpress) August 6, 2015
— Andrey Savchenko (@Rarst) August 6, 2015
However, while the latter was fun to make and a good learning experience for myself (made props Chris for contributing that AJAX validation!), the former has been maintained on WordPress.org for quite a while and certainly will be in the future.
Why would I want to switch site language anyway?
One word: tests.
When you work on a translated product (theme or plugin) and you’re responsible for testing the User Interface in more than one language, you will have a perfect use case and quickly come to appreciate a plugin like one of the ones mentioned above.
So, it’s a niche itch scratched with a niche solution. Made this purely out of need. If you think it helps you too, go ahead and have fun with it!