One of the important components of public diplomacy - in the past, in the future, and to even greater degree, in the future - is engagement with different ethnic groups in the host country/city. As public diplomacy strategy requires identification of groups and subgroups as target audiences, in order to try and influence them, the multuethnic environment provides good definition of such groups. The modern day communication, including social media, could serve as a tool for broa outreach effort as opposed to the limired outreach to formal heads of the ethnic groups if they exist.
For many years diplomats were aware of the importance of leveraging the influence of ethnic groups. The ethnic communities that originated in the country of a diplomat were obvious and central targets in an effort to improve the image the country they represent inthe host country. Thus, for example, Italian Americans wanted to help their country of origin on US soil, and of course Jewish Americans are involved in promoting Israel's interests. No doubt, the nationals of your country that live in the countries were you work as a diplomat, could be important facilitators in promoting interests of their home country.
However, the whole idea is to engage with other ethnic groups. This presents considerable challenge for diplomats as the language, the culture and codes of conduct of these groups could differ from those of the country they now live.
It is easier to bring examples from personal experience. During my post in LA, I saw first hand the importance of this dimension of public diplomacy. Our involvement with the communities from Latin American or Asian countries was always accepted as the right thing, and even gave some exposure in the media: http://articles.latimes.com/2007/dec/07/local/me-asiajews7
And why will we see more of that multi-ethnic environment, and not only in LA? Just look at the migration of people from country to country. Wherever you go in the West, you will find this phenomenon.
Two more things to mention - first, like everything in diplomacy, the engagement with ethnic groups requires higher level of sensitivity and deliberation. Not always it is possible to engage with the group if your country is in a state of animosity with the country of origin. Second, the real and effective engagement of the diplomat with the ethnic groups is possible in democracies. In authoritarian or limited democracies this kind og engagement is less effective and limited in scope, and even could be perceived by local authorities as subversive activity.