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If you don't yet have a Launchpad username, you'll need to sign up for one. Join the OpenLP Development team, which has a mailing list attached to it, and you will be subscribed to it automatically. Important matters should be discussed on the mailing list.
The old mailing lists on SourceForge.net have been discontinued.
We're using Transifex to translate OpenLP. You need to register there. If you don't see your language there, e-mail the translate mailing list above and ask for it to be added.
Once you've got an account in transifex, you'll want to start translating, but first rather read the Guidelines.
For a short overview about the different GUI elements you might want to have a look on Qt's widget gallery