If you are looking for an EXIF library that is well-designed, written in C, supports loading, editing (!) and saving (!), and supports internationalization by design, libexif is what you are looking for.
I still need help regarding manufacturer-specific tags. I imagine this library to be used in various programs (both with GUI and command-line tools), thus reducing the need for reinventing the wheel. If you already have an EXIF implementation, please look into contributing to this library and using it instead of your own implementation.