Learn Croatian: Budala je onaj tko vjeruje u takve laži. - A fool is he who believes in such lies.

Onaj tko translates to 'he who' or 'the one who'. In Croatian, it is much more natural to use the demonstrative pronoun onaj as an anchor in the main clause before starting the relative clause with tko. While you might occasionally hear just tko in casual speech, the onaj tko structure is the standard and expected way to express 'whoever' or 'the person who' in these types of sentences.

Budala is grammatically feminine. Why does the sentence use the masculine onaj instead of ona?

Although budala declines as a feminine noun because it ends in '-a', it is a noun used for a person of any gender. Because this sentence makes a general statement about 'anyone' who believes such lies, Croatian defaults to the masculine onaj tko to mean 'a person who'. You would only use ona koja if you were explicitly referring to a specific female.

Why is laži in the accusative case after u? Doesn't u usually take the locative?

The preposition u takes the locative case when describing a static location. However, when paired with the verb vjerovati to mean 'to believe in', it always requires the accusative case. Here, takve laži is the accusative plural.

What is the difference in nuance between using takve laži instead of te laži?