How to seamlessly switch between spelling languages in Mail?

On Mac OS X Snow Leopard (and maybe Leopard), the spell checker is smart enough to detect which language you're using.

Well, YMMV, personally I'm mostly writing in French and English, and I just have to type the 4 ou 5 first word, and Mac OS X automatically detect which language I'm using.

It seems to detect at least by paragraph, but it could also be by sentence, I haven't fully tested.

So try to set your spell checker language to "Detect automatically" and see if it's smart enough for your use.

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.