Please first search the existing issues, including closed ones, for a report that matches your issue.
When reporting a bug, provide details of the Fava version and browser used.
See here how to setup a development environment. Make a fork and submit your Pull Request. If it's a large change or you want some help to get started with coding, open an issue beforehand to discuss it or just ask about it in the chat.
Please contribute tests as well. See the tests/ subdirectory for existing
tests, which can be run with make test (requires tox).
You can contribute to Fava's translations at the Fava project on POEditor.com.