Short answer: It wasn't institutionalized, and it couldn't be.

Long answer:

Usually, what we see in states founded by nomadic peoples is a vast cultural and lingual difference between the ruler and the ruled. Settled civilizations have rooted, immovable institutions, and the ability to produce written material; both of which leaving the invading nomads in a conflict of culture that they can't even have the faintest hope of winning. If they wished to stay, they didn't try.

Mongols used the already existing institutions of governance. Sometimes they referred the actual management to Uyghurs, who preferred speaking Uyghur, a Turkic language. Kipchak dialects rose to prominence later on, which were also Turkic. In China, they again referred to the local elite when they could, and adopted the language when they couldn't. See where I'm going with this?

The Mongol Empire was multicultural, and the actual number of Mongols within it was nowhere near enough to justify enforcing Mongolian. Not that they had a reason to. The statesmen, the traders, the recently conquered populace, roughly half of the empire's army (Turkic nomads), all spoke something different.

Too add on to the above question: I want to point out that the Mongols were not concerned with changing their subject's culture, they just wanted money.

Before expanding on my addendum, I want to just provide an idea of just how vast the Mongolian Empire was (at it's height). Here is a [picture] http://i.imgur.com/5ZkzGnV.png comparing the Mongolian Empire and the Roman Empire. (The borders may not be precise, of course.) As you can see, it covered most of Asia: even today, governing that vast area would be quite the undertaking (and today, governments have the advantage of modern technology!)

Now, I want you to imagine a few things: you are the ruler of the Mongolian Empire. You have no telephones, internet (including email), or telegraph. There are no planes, trains, or automobiles. If there is a rebellion or other administrative problem, somebody would have to, quite literally, get on their horse (or walk) all the way back to the capital and hope somebody receives their message and responds in time (it is quite possible that the situation would have resolved itself before the message even was received by the Emperor.)

Your goal is to keep this vast stretch empire intact. How do you go about doing this?

Well first, you find some people who are intensely loyal to you and give them land and, in essence, autonomy. These individuals now rule in your place. They are responsible for governing your empire (it's obvious that you cannot do it alone.) In return, they receive rents, and send a portion back to you.

Now, they have their instructions and are dispatched. These people who rule over your empire face a second problem: how precisely are they to collect taxes? They need financial institutions, there needs to be infrastructure connecting commercial centers and localities to facilitate the flow of rents. There also needs infrastructure connecting the capital of the empire with main commercial centers. This is a vast undertaking. One should not that when I say infrastructure, I do not just mean roads. A road alone, while perhaps necessary, is not sufficient. One needs to defend the rental income as it goes from place to place. One needs somebody to take the income from place to place (who will not steal it) and they need a means of transportation. To overcome some of these difficulties, the Mongol leaders made contracts with commercial agents to handle some of these duties (transportation, collection, and investment of rental income.)

OKay. Perhaps, you've solved finance. Now, how about the law? Should there be one legal system or many? Who should enforce the law (the local leader or the monarch?)

Now, comes a more difficult task: rebellion. The question: does forcing a group of non-Mongolian people to partially or fully adopt Mongolian culture make rebellion more or less likely? Probably more. So, forcing them to speak a foreign tongue, likely isn't the best way to maintain one's empire. If there is a major rebellion that the local leader can't handle, one has to overcome many logistical problems and send an army for what could be hundreds of miles. You desperately want to avoid major rebellion.

Forcing people to speak Mongolian quite simply was not pragmatic. The time, effort, and money required to teach a group of people, some of whom may be somewhat hostile toward Mongolian rule, is not worth the benefit. It's easier to work with sympathetic local agents who are multilingual than to force vast numbers of people to learn an entirely new language.

What would the Mongols have to gain?

The other comments here support the idea that the Mongols were few in number and used other peoples for purposes such as taxation and accounting.

The Mongols, as author Harold Lamb pointed out, in his book Genghis Khan and the Mongol Horde (1954) were coming from a position of relative cultural poverty. Nomad peoples had no prejudice about marrying into the conquered peoples; and likewise no prejudice against absorbing the culture of the conquered. So in the West the Mongols adopted Turkic and Persian ways; in China they gradually were absorbed into Chinese culture.

To some degree, the Mongol mind was a palimpsest on which anything might be written.

The Mongol way of life hung on in remote parts of Central Asia and Mongolia, where it as well as its culture were not subject to cultural pressures as much.