A little while ago, Microsoft has introduced a new compliance capability to Office 365 called Communication Compliance to monitor language used and detect language abuse.
Well, while it has been quiet for quite some time, there is news on the Yammer front (yes, Yammer is still alive).
You can now define Yammer as a location to monitor by the Communication Compliance feature.
To define Yammer as a location to monitor, logon to your Office 365 Compliance portal (https://compliance.microsoft.com/) and reach the Communication Compliance blade (you may have to use Show all link before)
Then create (or edit) the policy you want to use to monitor Yammer and then edit the Locations