IBM i is actually the name of an operating system rather than the hardware. So, it is the successor to OS/400 rather than AS/400. The current hardware is IBM Power Systems. These can also run AIX and Linux.
There was also the name iSeries between AS/400 and i.
Regardless of its name, RPG has been the dominant language though it has also changed a lot. Today, Java may rival it but whether it is ahead of RPG will depend on whether you are counting new programs being written or programs in use.
Cobol is far behind RPG. PL/I used to be popular for the odd tasks that RPG and Cobol struggled with. C has been gradually taking over this role. Anything else is minor e.g. C++. Basic is (was?) available but I have never seen it used. I don't think that a Fortran compiler was ever available.
A complication today is that the OS contains AIX so it can run anything that AIX can. (The Portable Application Solutions Environment or PASE for short.) In this indirect way, Fortran would be available. Also, the hardware can be split into logical partitions, some may be running i while others run AIX or Linux.
I did a little more research and I found this: Programming Languages Supported by the IBM i Operating System
I see that Fortran and Pascal are supported but the EPM is very significant. I had forgotten about it.
OPM, original program model, is how the system used to work. It did not support any Unix like linking. One source file became one executable program and tens of thousands of lines were not uncommon.
I had totally forgotten EPM, extended program model, until I found that page. I probably dismissed it as a bad dream, it was the first attempt to support C. It allowed the AS/400 to claim to support C but it was close to unusable.
ILE, integrated language environment, was the next and more successful attempt to support C but it was a major change. It made the system much more Unix like.
Java came next and required yet further dramatic changes to the OS.
Later again, a full copy of AIX was buried into the OS. You can now run Python but it is just as if you are using AIX. A lot of new software is actually AIX including the JVM for Java.
In the link above, Basic, PL/I, and Pascal are labelled PRPQ. I had to remind myself what that meant. It is Programming Request Price Quotation. An approximate translation is: we will make up a price when you ask. It is not a standard price list item. These are products that IBM would like us to forget about.
Yet another addition
CL is Command Language. It is a rough equivalent to Windows batch files, Unix shell files, or job control language but it is more complex than any of those; for example it can even read a database file and handle a terminal. A further oddity is that it is compiled.
An obscure entry is MI: Machine Interface language. In the original program model (OPM), high level compilers generated an intermediate language called MI which went through a second stage compiler. This intermediate code was stored in the object and allowed the second stage to be rerun if the program was restored to different hardware (e.g. CISC to RISC). It was not widely publicised but it was possible to write MI and run the second stage compile. This gave access to deeper functions within the system. The new ILE program model uses a similar two stage compile for the same reasons but I am not aware of the ability to program in the new intermediate language.
There are / were further IBM only languages but I think that they are beyond the scope of this note.