GetSimple supports different languages for the admin panel. Download the latest language files for GetSimple from Extend.
Once you've downloaded the language file of your choosing, save it to the /admin/lang/ folder of your new installation of GetSimple. If you've already gone through the setup of GetSimple, you can change your language preference within your Settings tab.
Hint: As AccessKeys differ in different languages, you can define the AccessKeys which fit best for your language in the language-file. Search for the section For: include-nav.php in the language-file and wrap a single letter in <em></em> code there to define that letter als AccessKey for that function.
Example: <em>S</em> defines s as AccessKey for the Page-Menu in the german version.
For plugins conforming to the guidelines and GetSimple itself: you may use the Translate plugin - more information see Translation Tools.
There are functions and standards to internationalize (i18n) plugins. Check them out here.
CKEditor also supports languages.