To add a language to Sublime Text, you use Package Control. You can do this by pasting the correct Python code (available on the Package Control site linked above), or by downloading the .sublime-package file (also available for download on the site). Once you set up Package Control, restart Sublime Text.
To install the Processing package from Package Control, follow the following steps:
After following these steps, you should be able to select Processing as a language. This will facilitate coding in Processing in Sublime Text.