The formula for communications skills for heirs seems to consist of a) knowing the languages of your country and b) add a significant world language, in most cases, English.
You can see this with the heirs in Japan and Malaysia and Brunei.
Henri, Grand Duke of Luxembourg, and his Latina consort, the former Maria Teresa Mestre, made sure their children, including their own Prince William, heir Prince Guillaume, enrolled in local schools to pick up Lëtzebuergesch, the local dialect.
Grand Duke Henri and his 4 siblings were primarily educated in French, until recently the official language of the country, and would have benefited from fluency in Lëtzebuergesch, since 1994 an official language.
The same can be said for the affable Albert II of the Belgians, who manages to speak French and Flemish/Dutch. In contrast, his heir, Crown Prince Philippe and his wife, Princess Mathilde, are often criticized for their more hesitant command of Flemish/Dutch.
In the UK, the resurgent movement amongst the Welsh, the Irish, and the Scots means there is more than just English to learn.
During the 1960s, Prince Charles spent time intensively learning Welsh before his investiture as Prince of Wales at Carnavon Castle on July 1st 1969.
With the results of the latest vote in the Scottish parliament and passing of the Gaelic Language (Scotland) Act of Apr 21, 2005 making Gaelic the official language there (with equal respect to English), it may behoove the Wales boys to learn the language. Given that they sojourn almost 10% of the year at Balmoral, they should have picked up a smattering.
I wonder if St. Andrews, the Scottish university from where William graduated with his degree, has a requirement for its students to learn even rudimentary Scottish Gaelic? It could be one factor in the ultimate unity (or breakup) of the United Kingdom?
With the Wales boys covered in the area of English, which other significant world languages should they pick up?
It is all well and good to learn Swahili for pure interest glass houses case, I admit to voluntarily enjoying Latin for 4 years but it wont be long before savvy people direct them towards languages with a wider practical use for speeches and visits to the UKs most significant trading partners.
Thanks to Mrs Thatchers legacy, the UK positions itself as European, not Commonwealth. William and Harry could have their pick of languages from throughout the EU: these include Spanish, German, French, Italian, Russian. Or perhaps those languages from developing markets like Arabic, Chinese, Korean, Japanese, Hindi. (Any of you practising yoga already know Adho Mukha Svanasana).
To enjoy the full robust flavour of Hong Kong martial arts movies in the Jackie Chan canon, Harry would be advised to pick up some Cantonese!