Anyone can help by suggesting new translations here.
Once you've suggested a new translation, a GlotPress validator will review it. They will then approve, reject, or change your suggestions. Once your changes get approved, they are automatically shipped to all Jetpack users using WordPress in your language.
You can find detailed GlotPress documentation on this page.
You'll find options to create a .PO file and export translations at the bottom of each language page on translate.wordpress.org.
Some Jetpack modules, like Comments, Subscriptions, or Likes, are hosted on WordPress.com. Thus, you will have to look in the WordPress.com GlotPress project to find the strings to translate.