You can check the spelling of a message that contains more than one language or dialect. For example, when Spell Checker pauses at a word in another language, you can specify the language that Spell Checker should use, then resume spell-checking the message.
In the Compose window, click Tools > Spell Check.
Select the language from the dictionary list, then click Done.
Spell Checker uses the language you specify until you complete this spell-check session.