Subversion is the online program used to track changes and upgrades to WordPress for developers. See Using Subversion for more information. There is also an online browser for the WordPress Subversion repository.
Definitely! See WordPress in Your Language for information on existing translations, and Translating WordPress to find out how you can help if your language’s translation is missing or needs improvement.
Yes. WordPress Coding Standards has a list of coding standards used by WordPress core developers, plugin developers, and theme authors.
See How does code make it into WordPress.
See Reporting Bugs.
Check out Contributing to WordPress to find out what you can do and how to get involved.