The first question is: Is this novel still under copyright. This depends on the place and date of publication. In many countries copyright now lasts for 70 years after the death of the author. Some countries use different rules. In the US, a work published before 1978 is in most cases copyrighted for 95 years. Works published after that are protected for 70 years after the death of the author. More complex cases are detailed in this well-known chart.
Who holds the Copyright?
For most novels, copyright is initially held by the author(s). The author may sell or give away the copyright at any time. If the author does not transfer the copyright during life, it will pass with other property st the author's death. It may pass by will or by default (intestate) inheritance in the absence of a will. The author's surviving spouse children, if any, are often the heirs to any copyrights, but not always. Authors with many works still in print may have s "literary estate" set up to handle their copyrights.
Works with multiple co-authors usually have the copyright shared between all authors; in equal shares unless they agree otherwise. Any one copyright holder may license a derivative work such as a translation.
A would-be translator of a work still under copyright must find the copyright holder(s) and obtain permission. If the holders cannot be found, no permission can be obtained, and any translation would be an infringement of copyright. The copyright holder could sue for damages after publication. Damages may be quite substantial in some cases. This varies by country and by the facts of the case.
In the US The Federal Copyright Office will search its records to try to determine who holds a copyright. They charge a fee for doing so, and success is not guaranteed. Other countries may have similar services.
There is no automatic or guaranteed way to find who now owns a copyright. In some cases the owner does not even know that s/he owns the copyright. If the holder died without heirs the copyright may belong to the government (state government in the US).
Research into the author's life may reveal probable heirs. An obituary may list the author's children, if any. Finding them and asking is a reasonable place to start.