The original sentence translates into: He speaks Chinese very fluently One way to change it is 他说得一口很流利的汉语。 Comparative translation is : He speaks some very fluent Chinese. 一口 is the quantifier for 汉语 that should be used in situations like this

他说得很流利汉语 => 他说汉语, 说得很流利 From my experience, the 得 pattern is often this: "He speaks Chinese, speaks very fluent" You cannot put an adjective in front of the noun without using the "de" adjective particle after the adjective, which you've omitted.