First of all install the necessary languages using the Language Manager of Joomla. This is independent from this component and a requirement for the next steps. To translate the gallery into your language you have several options. Let’s assume you want to have the gallery translated to Spanish with the locale es-ES. The following options are available.
With each new version it might be necessary to add new translation keys to your custom file. If you choose option 1 this would be possible without touching any file in the file system manually. You can use the Joomla back end to add the missing keys. Starting with 2.6.4 each installation or update will delete translation files from /language/xx_YY/. If you added your changes there make sure you migrate them to one of the options mentioned above.
If you want to contribute or need a blue print for your own translation, you can got to my repository for custom translations: https://github.com/svenbluege/eventgallery-translations. Using this way of translating, you can easily create an installable language pack.