Just learn Fortran. The output of machine-translated code may well be functionally correct and suitable for compilation and execution, but it's going to be really hard to understand and maintain. (Just look at what generated code targeting a single language, like the output of a GUI builder wizard, looks like.) In particular, while Matlab is built on Fortran, its idioms at the M-code level are different enough that it would be pretty incomprehensible. If you already know C or any other Algol-like language, picking up Fortran is not that hard. And the idioms and features that are particular to Fortran – that is, the new stuff you'd have to learn – are probably going to be especially weird and incomprehensible when sent through a translator.
The one translator that might actually be useful is a Fortran 77 -> Fortran 90 translator. The modern '90 dialect would be easier to learn and more succinct, and since the translation is within the same language family the output probably wouldn't be too ugly.