First of all: thank you for contributing to Firefly III! It is always nice to get some help!
If you find a bug, please take the time and see if the demo site is also suffering from this bug. Include as many log files and details as you think are necessary. Bugs have a lot of priority! You can send the bug info to me by email or by opening a ticket on GitHub.
When you want to fix a bug yourself, feel free to open a pull request.
Opening a pull request with some changes is always a possibility as well. Keep in mind that I may not be able to implement all requests I get.
There are two main projects for the translation of Firefly III. They are hosted by CrowdIn, where you must register an account to help out.
If you see a spelling error, grammatical error or a weird translation in your language, please join the correct projects. There, you can submit your translations and fixes. They are synced to GitHub every so often and included in each new release.
The wiki page on GitHub has some extra instructions for translators.