How to seamlessly switch between spelling languages in Mail?
I have just spent some time with Mountain Lion after I added a custom dictionary (Afrikaans) to the ~/Library/Spelling folder (.dic and .aff).

Findings:
- If I set the "Spelling" in "System Preferences -> Language and text -> Text" to Afrikaans, the Afrikaans spell check works great.
- If I set the "Spelling" to "Automatic by language" detection between English and Afrikaans it does not work correctly. It never recognises Afrikaans, checks everything as English.

So in mail.app the only workable option seems to be to set it up to check email upon sending, and to select the language at the bottom of the pop-up box.

Alternatively buy Postbox, where you can install a Thunderbird dictionary and select the language manually. But Postbox is not doing retina displays (yet) - it looks awful at the moment on retina.