I have a proverb in my native tongue saying something like "there is no cat chasing fish for God" which implies that anyone who does anything that may seem beneficial to you, is doing it for themselves. What is the equivalent proverb in English?
In the UK there's "You don’t get owt for nowt." (owt = ought = anything/something, and nowt = nought = nothing).
Plus, as Wayfaring Stranger comments, There ain't no such thing as a free lunch.
Not a direct equivalent, but very close meaning as well, like the one of Wayfaring Stranger
It is a proverb that means that everything costs something, and anything that appears to be free must be deceptive (grammatically incorrect, it would use anything and not something, but it's idiomatic)
I would argue that although there may be expressions that approach the meaning of the expression that you have supplied there is no equivalent in English that is as good.
The expression as you have translated it from the original Tunisian is, quite simply, delightful.
I would respectfully suggest that you modify it in speech to say, "There's no cat chasing fish for God" or "There's no cat fishing for God"
This expression as it stands is a delight for the following reasons:
Finally, it is worth pointing out that English is a mongrel of a language that sucks up other language and idiom from all over Europe and the Rest of the World, from the grammatically suspect, "I'm loving it!" to words such as 'shampoo', 'pyjamas' and 'galore' and also including the sentence structure of languages that it has supplanted.
So I would implore you, for the sake of this great expression, use it as you have translated it and when quizzed by those who ask you about it, tell them it's an old Tunisian saying.
It's likely they will thank you.