Chrome: Automatically detect language for spell checking
This is being tracked in issue https://code.google.com/p/chromium/issues/detail?id=5102. You should "star" the issue to be notified of progress.
There is also this one:https://code.google.com/p/chromium/issues/detail?id=1834And this design document:http://www.chromium.org/developers/design-documents/advancedspellchecker
The design looks a bit overkill, I hope they implement something simpler sooner.