This article lists common questions for WordPress core developers, as well as developers of WordPress plugins and themes, and refers you to articles that contain the answers. There is also a list of resources at Developer Documentation, which might be of help.
Check out Contributing to WordPress to find out what you can do and how to get involved.
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.
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.
Problems with plugins are best reported to the plugin author (visit the plugin’s home page to find out how to contact the author). For plugins that are included in the wp-plugins.org Plugin Repository, you can also report bugs in the wp-plugins.org TrackTickets site.
Most plugin and theme authors choose to license their plugins and themes under the GPL. Also, keep in mind that if your theme or plugin incorporates code from another theme or plugin that uses the GPL, you will probably need to license your theme or plugin with the GPL as well. If your theme or plugin is completely independent of other WordPress code, themes, and plugins, then you can probably license it however you would like.