Probably whatever language you're using for the rest of the project. Of course it's most straightforward to do in C but most higher level and/or scripting lang

Probably whatever language you're using for the rest of the project. Of course it's most straightforward to do in C but most higher level and/or scripting languages provide some sort of API for dealing with binary data because they often need to interface with low-level C code. For example Python has the struct library which is fairly easy to use.