I think "the chickens are yelllow" is also a valid translation.
IMO a translation of this sentence in isolation using 'the' is incorrect. Unless the word "the" is required for an idiomatic translation, or required by a specific context, you should insert it into the English only if there is a corresponding demonstrative adjective (e.g., to/ty/ten) in the Czech sentence.
To put the matter another way, with no further context, the Czech sentence here seems to be a general statement about all chickens, not about some specific chickens or some chickens that have been previously mentioned.
True, you could dream up a conversation/context in which the Czech sentence here would correspond to an English sentence beginning with "the". But why go through those gyrations, when you can translate the sentence we are given here simply as "chickens are yellow"?
chicken can also be uncountable and have the form "chicken" in plural
But not here, because both the Czech noun and the Czech verb are in the plural.
When uncountable, it takes a singular verb. For example, "Chicken is an important food ". So I would not say that it is in the "plural" here, but rather that used as a mass noun, it is singular.