The best is always to make tests on a local installation of your website rather than make plugins tests on a production site. Otherwise the minimum is to backup your DB first.
For what concerns Polylang, posts in translated language are standard posts. The same for pages, categories, post tags... Polylang adds taxonomies and metas to filter content to avoid mixing languages but does not mess your posts.
When you de-activate the plugin, all data are kept intact in DB. When you uninstall the plugin (with the red delete link in plugins table), all data are removed from DB. That's what all plugins should do.
I can't garantee there won't be any plugin/theme conflict. That's why a test on a local install is always better.