Make language specific ContentPlaceholders.
I agree with chris.brandsma's suggestion wholeheartedly.
If you could put the directive in the ContentPlaceholder, then have that used to prepopulate the Content element with a "language" attribute. I would call that a solution.
I would rather prefer to be able to specifiy what type of IntelliSense I would like to use at any single place in code (ie. by beeing able to specify some Regex rule where CSS IS should be used, another rule(s) for JS, etc.