You may think the Czech president is stupid or indeed says things that are stupid, but if you want to express this to a English speaker make sure you say it correctly.
Firstly, we don’t say stupidities. It is an uncountable noun. We sometimes say the word stupidity but it is often connected with the word ‘own’. We say something like, “I failed the exam but it was my own stupidity. I misread the last question.”
More often than not though, we just use the adjective, rather than the noun. We would say, “The Czech President is always saying such stupid things.”
A similar mistake is to say ‘nonsenses’. Again, this is uncountable. In this case, we would use the noun form and you could say something like, ‘He talks such nonsense.’