1. Nope. Unless you've hacked WordPress files (and you'd never do that right?) you have nothing to fear for your data.
2. If the plugin is already has i18n support and the language files exist then possibly. But you may want to try on a test installation first.
3. See answer #1. ;) Your data is in the database and installing different versions should not hurt a thing. Now you may encounter encoding issues (possibly) but again setup a test installation and try there first.
Current full backups and knowing how to restore your database as well as your files is your greatest safety net and if I'm totally off base then you'll be glad to have them.