I'm using Polylang to translate my originally Dutch site into English. It works mostly very very well. A few strings don't translate however: site tag line and the post metabar. I contacted the developer of Polylang and he told me the following:
If the theme or a plugin uses the standard WordPress function to display the site title and the tagline (as of course themes developed by the WordPress team do!), then Polylang knows that it must translate these strings and so you can translate theme in Settings->Languages->Strings translations.
However, if the theme or a Plugin uses another way, then Polylang can't know by itself, so the theme or the plugin must inform Polylang about the string to translate.
This is done by the functions pll_register_string on admin side and pll__ . There are several examples in the forum on how to adapt themes or plugins (as well as some links in the doc).
When he gets to 'This is done by…' I've lost him. So hopefully someone has experience with this and can point me in the right direction.