Ask HN: How would you create a utility to open a custom file format?

What's the easiest language to use if I need to convert an arbitrary file format into variables in memory? (assuming there is no existing library)

Are there any languages that really lend themselves to this problem?

The data is very heterogeneous (think: underspecified file format spec with multiple independent vendors) with a header that is lots of text labels, and a body that is numeric data of variable bit depth, numeric types.