If you're looking for a place to start, the bug tracker is full of bugs to fix and crazy ideas to implement :-). Beware that tickets with the label 'to study' must be discussed further first before starting writing code.
If you need help for anything, don't hesitate to ask either on the mailing-list or IRC. We will gladly do our best to help you.
Before every important release of Paperwork, there is a testing phase (release candidate). This phase lasts about 1 month. While there are some automated tests, most of the tests are unfortunately manual tests.
In a perfect world, Paperwork tests should be run on all possible platforms and distributions.
The best time to translate Paperwork is when it enters testing phase: Before each important release, there is a test phase of about one month. This phase is announced on the mailing-list.
Note that the UI will keep changing after your first translations, and they will need to be updated. Since it's better to have no UI translations at all than incomplete UI translations, unmaintained translations will be removed.
Paperwork intends to be as simple to use as possible. To make its installation as simple as possible, Paperwork must be packaged in GNU/Linux distributions (Flatpak is still to hard to use for my grandmother).
The following libraries and program should be packaged: