A case can easily be made for leaving out "the." Let's say you were in a race and everyone else finished before you. If you were describing the result, you might more likely say "I was last!" than "I was THE last." If you wanted to expand your comment, you might say either "I was THE last person to cross the finish line" or "I was last to cross the finish line." There are probably other examples, but this one jumped out first.
There is, however, nothing wrong with "You weren't THE last." It's just a different usage. Example: "You weren't THE last to waste a lot of money trying to win the lottery!"
But I don't know whether the Czech sentence can accommodate both usages.
Very interesting, I run into the same thing in Duolingo Korean. However, it doesn't work in English, at least not the way it supposedly does in Korean. You can be one of the last, or even one of the very last. I wouldn't even hesitate until I got to the lastmost, doubly superlative, and that's not even turning up in the dictionary except as https://www.lexico.com/definition/-most . . .