The Polylang plugin is not currently supported but may be in the future.
There are a few hack-ish solutions to this, however, my first question would be: for what reason do you have a single language Magento installation (single store) and then want a multi-lingual blog?
The easiest solution would be to create a different store view for each language you have and then, using WordPress Multisite, assign a different blog to each store view. This would give you as many different languages as you need.
The only issue I can see with this would be that when you're viewing a post, if you change store view, you won't be taken to the equivalent post on the new store view as the URLs would not match due to the language differences. To fix this, you could add a RewriteRule to your htaccess file that always redirects to the blog homepage when you're changing store while viewing the blog.
If you have to use a single Magento store and still want a multi-lingual blog, it would be possible using the Root addon to create as many different blogs per store as possible. These blogs would all need a different URL, but this could be done using URL's such as:
- en/blog/
- de/blog/
- fr/blog/
- es/blog/
If this sounds like it could work, let me know and I will provide more information.