Polylang 1.4 introduced a new metabox with a dropdown list to show available posts (instead of post ids) to choose translations. I believe that this change was quite welcome as it was easier to use and I guess users having not too much untranslated posts are happy with it.
This however introduced a major issue for big websites. The metabox does not scale for a site having ten of thousands posts. And for these sites, it is just impossible to use Polylang 1.4.x. Even if such big websites are probably rare , I cannot let them stuck to Polylang 1.3.1.
I thus decided to rework the metabox again and replace the dropdown list (currently displaying *all* available posts) by an autocomplete input field (just like the one you are already familiar with to get post tags). The same kind of autocomplete input field replaces the old dropdown list to choose translations of categories and post tags.
This version also comes will less visible improvements such as better error management on Polylang admin pages and better compatibility with object cache plugins.