You'll have little problem learning C, as it closely parallels J for most conventions.
You may have a harder time with Fortran, especially considering if its an older version. F9x should prove safe enough for a person experienced with J, but if your using something like Fortran 7x, then I definitely recommend obtaining a good programming guide. My scientific application development classes required F77, and it was not too fun having to deal with the archaic code conventions.
Robert Richmond