For a long time now, Polylang attempts to download the WordPress translations files when a new language is added. That should work pretty well for all translations available on the WordPress svn repository. However, the update management was far from being perfect. It fired only just after a WordPress update and so could not work if the new translations files were not available at that time.
Also, now that the WordPress translations are managed on http://translate.wordpress.org/, less translators update their translations on the svn repository and thus, Polylang cannot find translations to download.
Fortunately, WordPress 4.0 introduced a new way to install and update translation files, called language packs. The main goal of Polylang 1.6 will be to take profit of this new WordPress feature to manage multiple language packs. Thus the translation files will be updated in a much clever way and will work for all language packs managed by the WordPress team.
Polylang 1.6 also introduces a few other features such as the possibility to translate a text widget in the strings translations panel (up to now you needed to use one widget per language), the support of opengraph language and translations metadata for websites running Jetpack or WordPress SEO.
As usual, I need you to test this new version, so don’t hesitate to download Polylang 1.6 beta1, test it and report bugs in the support forum.