As Perch supports multiple languages, apps too have the ability to do so. Language files are created in perch/addons/apps/my_app/lang.

During development of your app you should:

As you create your app, text strings from the interface will be added to a new translation file inside the lang folder.

Strings passed through PerchAPI_HTML and PerchAPI_Form are automatically added to the translation. If you write anything to the page directly, you should pass the string through PerchAPI_Lang::get().

$Lang = $API->get('Lang');
echo $Lang->get('Welcome to my app');

The Lang API reference has full details of how the languages functions can be used.