So the answer to the question in your title is:
With 雄 and 雌
Which are the words used in Japanese to express the gender of animals.
However, I think the real question here is:
Why is there information in the resultant translation that does not appear to be in the original sentence?
First of all, let me confirm that nowhere in the original Japanese sentence does it specify that the cat is female. Unfortunately, this problem (where one language has more information than the other) is fairly common on a lot of translation sites that give individual sentences. Realistically, I think this happens for one of two reasons:
- Because adding extra information makes it more natural in the output language. In the case of your example sentence, the translator may have simply decided that because no one refers to their cat as "it" in English, they should arbitrarily assign it a gender. Translating from JP -> ENG without sufficient context also often results in guessing about plurals.
The sentence was translated as part of a larger body of text which
contained the extra information in a nearby sentence, but that
information moved around when translation happened. Consider
Which is roughly
My cat is such a baby, he follows me around wherever I go. (He does this) even though he's male.
Any half decent translator is going to put a "he" in the first sentence, despite the fact that there is no information about gender in the original Japanese until the second sentence. This is a slightly contrived example, but hopefully it clarifies one way this can happen when you only see one sentence at a time.