Cisco doesn't specify whether or not Spanish language AD is compatible. They only note "You can install the user agent on any Microsoft Windows Vista, Microsoft Windows 7, Microsoft Windows 8, Microsoft Windows Server 2008, or Microsoft Windows Server 2012 computer with TCP/IP access to the Microsoft Active Directory servers to monitor. "
Be advised however that Firepower User Agent is being deprecated (no matter what AD language). As of 6.6 you cannot create any new User Agent identity sources. 6.7 won't even support previously existing ones in an upgrade scenario.
