About the "next" button issue, I reported it at the Comic Easel forum too. I think it would be a bug of that plugin, because the other buttons and functions works well with Polylang.
But I'm interested in the publication date (sync) of the new translations. I think the new translation must syncronize with the older published post. :/ (It's possible?)
Also a more easy way to find the posts in other langs, like the selector (dropdown)in the tags/categories.
BTW, awesome plugin. Keep up the good work! :)