There are two ways to define language diversity.
Both ways of assessing linguistic diversity, absolute number of languages and the percentage of the population speaking the largest language, are useful measures for certain purposes.
Source: WHY SHOULD LINGUISTIC DIVERSITY BY SUPPORTED IN EUROPE? SOME ARGUMENTS (T Skutnabb-Kangas)
While the European Union is committed to integration at European level, it promotes the linguistic and cultural diversity of its peoples. It does so by promoting the teaching and learning of their languages, thereby building bridges to greater solidarity and mutual understanding.
The European Union’s policies for linguistic diversity are designed to create an environment that is propitious to the full expression of all languages and in which the teaching and learning of a variety of languages flourishes. The European Union’s ambitious goal is that as many of its citizens as possible should speak two languages in addition to their mother tongue.
While recognising the emergence of English as the most widely-spoken language in Europe, the European Union also wants to make sure that this does not become, over time, a factor which reduces linguistic diversity within its frontiers. This is why the Commission has set a target of “Mother tongue-plus-two” for its policy on language skills.
Source: Languages and Europe