Hi,
1. Well, if you have caching on your website it could potentially happen. But you can clear your Joomla cache if you have any.
But normally, this just means that you didn't configure Falang according to their documentation and skipped some parts of it.
2. No. Most extensions don't remove their data in the database when you uninstall them. Imagine if you accidentally uninstall HikaShop (it happens) and you lost all the products, the orders, etc of your shop... So you sitll have all the data in the database and if you reinstall the extension, you'll still be able to manipulate the data.
And deleting the data in the database is easy, you can just go in phpmyadmin and drop the tables of the extension. That's a lot less likely to happen accidentally.