What makes you think so?
"I don't know whether to look" in double quotes (as a whole expression) has 1.5 million hits on Google, that's not bad.
There's even this mug: https://www.amazon.com/Whether-Vintage-Ceramic-Coffee-Black/dp/B08DXJ5WFG
Although "if to watch"would be a literal translation of the Czech sentence, it is incorrect in English. "Whether" should be used here.
In English, "if" could be used in phrases like "if I SHOULD watch" or "if I WANT TO watch," but those aren't good translations of the Czech version.