The IBM AS/400, formerly known as System/38, subsequently known as i (sic), is remarkable in being essentially the most future-proof of all the minicomputers, thanks among other things to the use of byte code to screen programs from the CPU; this was conceptually similar to Java and .Net byte codes, but instead of being an optional extra, it was the only format in which applications were distributed, so seamless upgrade of CPU architecture was guaranteed to be possible. As a result, when the eighties became the nineties and other minicomputers like VAX and Wang were dead or dying, the AS/400 was just hitting its stride.
The first half of this is a good plain-English overview; the second half is the best computer marketing video I have ever seen: https://www.youtube.com/watch?v=5pY6Xxptp9A&ab_channel=AndyK
What programming languages were most commonly used on this system in the nineties? Of course, it ended up supporting every language under the sun (no pun intended) just by virtue of being used widely enough for long enough, but which ones tended to be actually used at that time? Probably not Fortran; the marketing guy in the above video was asked how many MIPS the system could do, and he replied, what are you going to use the machine for, solving the Schrödinger equation, or running payroll? But COBOL and RPG would surely fit. Was there a culture of continuing to use these old languages, or of moving to Pascal, C and C++ that were popular on microcomputers? Did the Java trend come to the AS/400? Was it a stronghold of PL/I? Did Smalltalk ever have its day on that machine?