Create a new issue with as much description of the problem as possible. Include steps needed for reproduction if the bug is not obvious. It's also possible to submit bug reports anonymously to my feedback form.
The same as bug report. Well-explained suggestions are preferred.
Translations are stored in .resx files (almost all except for Resources.resx). It's suggested to use the ResxTranslator to translate these files. It will save you a lot of work. Most if not all of translators here use it.
You can translate by creating a branch, translating it, and then starting a pull request. If you are not familiar with git and GitHub, you can ask for a translation pack through my feedback form. It will contain everything needed. Once done, simply send it back to my mail.
Any modern version of Visual Studio should work. You might need to download download this and this library separately. Tests use the test framework included in new Visual Studio versions. Some of the tests require running as 64bit to pass, or some specific applications to be installed.