Ich kann [X language] is pretty much the standard in colloquial speech.

Ich spreche fließend Deutsch is correct.

The reason why it's not "fließendes Deutsch"* is that "fließend" is an adverb here that qualifies HOW you speak the language (you speak it fluently) and NOT that the German language you speak is fluent. In English, you can say either "I speak German fluently" (adverb referring to verb) or "I speak fluent German" (adjective referring to the language) but in German, the latter sounds strange/is not really used.