I suspect it is more the bias than the technical difficulty. You can you the same techniques used to predict words to predict the language they are in, especially if you limit it to a user-defined set of languages. I would be tempted to try it but I don't speak multiple languages so I would be a terrible test subject.
If think you could experiment with an interface that actually does to solve this problem and does auto detect the language:
http://translate.google.com/#auto
Once a language is detected it won't reevaluate the input, so you have to paste whole sentences in it. The implementation really goes above and beyond to guess the input, I think that's what you and other mean by matching several languages at ounce.
For instance "J'aime pas la philosophie du 'just do it'" will translate both french and english parts at the same time.
The thing is, 'just do it' is a ready made english sentence, but you could autocorrect it to 'juste doit' or 'jus doit', both of which are valid single words in french. There's no way to know what you really meant on the autocorrection front (we have to assume you made a mistake to suggest something), and you'll need an interface constantly suggesting correction that you'll dismiss 99% of the time. This feels like hell in a popup to me.
