Polylang translation of WP Booking Calendar

I've a WP website

http://www.chalupabukovec.cz

with Innovative theme

https://wordpress.org/themes/innovative

and I'm using Polylang plugin for Czech and English translation. Everything works fine except I'm really stucked on translating WP Booking Calendar plugin.

I know there is a automatical translation of this plugin for qTranslate but qTranslate didn't work for me as good as Polylang.

I've read a lot of topics here about string translation like:

pll_register_string($name, $string);

but I really don't know where I can find these strings what I'm looking for. Also there are some .MO/.PO files at WPbookingCalendar plugin directory which are used for translation but are these files anyhow useful for me right now?

Anyone any advice or experience?

Thaaank you :)