A trip to the redoubtable The Elements of Style shows that the word that often stands in for which. When we talked about this, the subject drew one pun after another, making both words sound as if they were the right choices. As you can see from the cover above, this book has been an essential through many generations of writers.
“The use of which for that is common in written and spoken language,” Strunk and White say in their book. “The careful writer, watchful for small conveniences, goes which-hunting, removes the defining whiches, and by so doing improves his work.”
That is a defining pronoun: “The lawn mower that is broken is in the garage” tells us which mower.
In contrast, “The lawn mower, which is broken, is in the garage,” adds a fact about the only mower in question.
To put it another way, make the whiches in your writing prove their existence. Much of the time, that will serve nicely. It’s a choice which is more conversational in tone, too — and that might be closer to your native voice.