Dear chlodny,
Just saw your comment yesterday. We are very sorry what happend to you. To be honest, the plugin was released to early to the public. We have been mainly concentrating on making the plattform https://www.webonary.org a good service and haven’t invested much time in making documentation available for people who want to install the plugin themselves. This is going to change soon.
As of today we released version 2 which solves the problem you had. It won’t delete any posts when uninstalling the plugin. It now puts the posts in a category “webonary”, so when deleting posts in the plugin area, it will just delete those posts. Nevertheless, Webonary doesn’t work with blog posts (we just added a notice concerning that on the import page). Webonary is not a traditional WordPress plugin, since the dictionary entries are the “blog posts”. You can however create “Pages” if you want additional text on your site. If you want blog posts and a dictionary, you might want to create a multisite WordPress installation.