You may use the Translate plugin for providing new language translations for GetSimple and plugins (as long as they conform to the internationalization guidelines):

1. Install the Translate plugin
2. Go to the Plugins/Translate view
3. Select a plugin or GetSimple, then a source and target language
4. 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
5. Start translating for real

Transifex is an online platform for collaborative translation.

Proposal for the use of Transifex for GetSimple and plugins: Transifex