Per-application selection of correct spell checker language on Mac?
You can't set it per application, but you can set it to support multiple languages and detect them automatically.

To do so (in Lion - although I've been using this at least since Leopard), open System Preferences, go to Language &amp; Text, select the Text tab, set Spelling to Automatic by Language, then open the Spelling dropdown again, select Setup... at the bottom and check the languages you want to enable.