If you often compose mails with different languages and use spell checker quite often, GMail cleverly does not ask you in which language the mail is written, nor you need to specify one. You write in English, then English spell checker is used. You write in German, then German spell checker is used. Seems trivial, still I was happy when I realized this for the first time.
On related thing, GMail seems to use Sophos for its antivirus.