You can probably use either “which” or “that,” and the meaning would be the same. Considerations: “Which” can only refer to things, not to people. A book is not a person, so we’re OK to use “which.” “That” can only be used in a defining / restrictive relative clause, not a non-defining / non-restrictive relative clause. Without the full sentence (we need an independent clause), we cannot be sure which type of relative clause is meant, but the lack of a comma makes me suspect it is a defining relative clause, in which case we can use “that.” By “defining relate clause,” I mean that the clause is necessary to identify the book (The book that I’m trying to translate is very interesting), as opposed to simply giving us additional information (“The Shining,” which I’m trying to translate, is very interesting.) If it is a defining relative clause, we have another option, since the relative pronoun refers to the object, not the subject. In this case, we can omit the relative pronoun entirely “The book I’m translating….” So, if it’s a defining relative clause, you can use any of the three options, although if it really is a defining relative clause, it would be somewhat better style to use “that” (or no pronoun) instead of “which.” On the other hand, if you really can’t figure out which kind of relative clause it is, you can safely use “which.”