In normal cases ownCloud will automatically detect the language of the Web-GUI. If this does not work properly or you want to make sure that ownCloud always starts with a given language, you can use the default_language parameter.
Please keep in mind, that this will not effect a users language preference, which has been configured under “personal -> language” once he has logged in.
Please check settings/languageCodes.php for the list of supported language codes.
<?php "default_language" => "en",
This parameters can be set in the config/config.php