The site title and the tagline are part of the general settings of your WordPress. You can translate these texts using the “String translations” feature of Polylang:
Languages -> Strings translations
Did you translate the site title but it is still displayed in only one language? In this case you can check if the themes header is using the correct WordPress functions to get the title/tagline (wp_title() or get_bloginfo()).
What did we miss? Write us at development[at]supertext[dot]com with your question about Polylang and Supertext.
Titelbild via MaxPixel (CC0)