If you're not a localization leader, you're going to be able to suggest translations, and localization leader for your language will check those later and either accept or refuse those suggestions. We're advising all our contributors to contact Mozilla Localization Team for their locale(language), and coordinate with them. Localization teams contact pages can be found at https://wiki.mozilla.org/L10n:Teams . Sending an email to localization leader would be good start.
In case you can't find your language in Mozillians project on Verbatim, that means nobody yet volunteered to start working on it. But that can as well be you. You can request for Mozillians project to be added in your language via Webby application(using your LDAP credentials), or you can ping Milos on irc.mozilla.org, channel #l10n and #mozillians
And last but certainly not the least, we've recently created a l10n-calendar which holds info on when do we add new strings to Verbatim and when do we release.
We would love to have you on board!