Translation:Czechia and Slovakia are to the south of Poland.
I am native AmE and it is very normal to say "THE Czech Republic," at least where I've lived.; in fact, I don't think I've ever heard it without "the." :-) "Czechia" is a newer option, so it probably comes down to which a person prefers to use, at least for now. And you may be absolutely right about having gotten used to CZ natives leaving out "the"!