Why is "the" needed here. Ja by jsem to prelozil takhle "You weren't last."
"já bych" - by jsem does not exist in Czech.
English requires definite article for superlatives, which "the last" is. There is only one, unique last. Otherwise it is not the last.
The definite article in this particle case is most certainly optional. "The" may be hypercorrect, but it is not necessary. In fact, to be hypercorrect I might be inclined to add "one" at the end as well.
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.
I agree with mr bass that there is no need for 'the' hear. Certainly i can find no credible source in UK english to support Kacenka's statement. In this context 'the' sounds strange to my english ears.