It's not decline, it's just a tedious task.
You're having trouble finding the motivation to deeply understand the original API.
So, cheat your brain with busy work:
- find typical use cases (hopefully the original was written around use cases)
- use it in the original language, write toy programs
- look for concepts and symmetry (i.e. open/close, add/remove, handles, whatever) that reduce the cognitive load
- mock a rough translation in the target language (it's not final code, hacky works)
- slowly port your toys
- watch for gaps... wash, rinse, repeat.
In the meantime, sometimes finding something mechanical to pursue that moves the job forward may help.
For example, mocking all API calls in the target language, following the original structure. TDD? Build a tool to make the target API shims?
It takes time to absorb something that's not calling you, but you can get there.