Say I convert an AGPL-licensed library from Java to Kotlin using the IntelliJ code conversion functionality, and change the code from there. Can I then use this converted library in my closed source commercial project without violating the AGPL license?
In my opinion, no. Machine translation doesn't change the licensing terms on a work any more than converting a Hollywood film from .avi to .mp4 strips the copyright and permits redistribution.
So the machine-translated version is just as AGPL-covered as the original. You then modify it, making a derivative work, which is required by AGPLv3 ss6 and 5c to be distributed under the terms of AGPLv3. Proprietary, closed-source redistribution would violate that.
Definitely no. The license is not rendered void simply because you take a programme and translate it into another language; translating existing source code into another language is similarily a derivative of the original work as is adding any feature or other alteration.
See also the license text which reads
A "covered work" means either the unmodified Program or a work based on the Program.
In essence this is the same question as this some time ago concerning GPL v2. However... IANAL.