You may use the Translate plugin for providing new language translations for GetSimple and plugins (as long as they conform to the internationalization guidelines):
Go to the Plugins/Translate view
Select a plugin or GetSimple, then a source and target language
Translate a few texts than try saving - if it doesn't work, you probably need to create the directory backups/translate and make it writable and/or make your language directories writable
Start translating for real
Transifex is an online platform for collaborative translation.
Proposal for the use of Transifex for GetSimple and plugins: Transifex