The other site has no XLIFF file for that domain and language under translations/, so the key prints itself. Then: the call names no domain and falls back to the admin interface's domain; the interface language is chosen from the cdna_locale cookie, then the account default, then the browser, so a site that never sets the cookie shows the default language; or the site is Twig and the templates use a Smarty modifier, where Twig needs __.translate(key, params, domain).