When I was a ten year old kid in 1973 my school did a tour. I remember looking at some programs running on green-screen terminals, and the guy doing the demo typed in commands using the keyword FOR so I assume it was all Fortran back then.
Numerous programming languages, including basic, algol, etc used for loops. Notably absent from the list of languages which used FOR as a looping construct was Fortran, which used 'do' for looping, and cobol, which used 'perform'. So whatever languages it was, it wasn't Fortran.
On a lot of systems the fortran compiler is often named fortran and usually aliased as for and fortran in the shell. I know as well on VMS, RSX, and RSTS you, at a minimum, just need to type the first 3 chars of a command for DCL to figure out what it is.
You can get a rough idea if you watch this...
https://www.youtube.com/watch?v=1MGjFKs9bnU