This is not a lot information to go on. That makes it quite hard to help you with your problem. I suggest you share what you have (main.cpp and CMakeLists.txt) so that others can try out if they can replicate your problem.

@Hitokage OK looks like the new lupdate version needs additional info about the file's language so I had to add something like this: <TS version="2.1" language="cs_CZ"> Works now!