LANGUAGES configures accepted languages. gestor_config() removes a valid language prefix from the path (/en/...) and adds it to root URLs; without a prefix, a valid language cookie can select the language. The router queries pages whose language matches the active language. Available translations of a page are determined by querying matching pages.
Resources for different languages are separate files under resources/<language>/ and separate database records. A missing resource is not translated automatically. System variables are queried for the active language; interface text should have corresponding resources in each language. See resources and global variables.
gestor/bibliotecas/lang.php implements a separate JSON dictionary with __t(), which returns the key itself when a translation is missing. This dictionary does not replace compiled variables resources; its expected default dictionary file is absent from the current core. Use per-language resources for site and admin content.