In normal cases, ownCloud will automatically detect the language of the Web UI. If this does not work as expected, or you want to make sure that ownCloud always starts with a given language, you can use the default_language configuration parameter.
This parameter can be set in config/config.php
'default_language' => 'en',
Keep in mind that this will not affect the language preferences of users, which can be configured underonce they have logged in.
More supported languages can be found in directory <ownCloud_root>/settings/l10n. List all files with ls *.js. The language code to be used is the filename without extension.
en_GB.js --> en_GB
Refer to Wikipedia for a match of language code to country.